The following is from the web page Luther, Exposing the Myth, under the heading "Marriage and Women":
"In spite of all the good I say of married life, I will not grant so much to nature as to admit that there is no sin in it. .. no conjugal due is ever rendered without sin. The matrimonial duty is never performed without sin" [Weimar, Vol 8. Pg. 654].
In other words for Luther the matrimonial act is “a sin differing in nothing from adultery and fornication.” ibid. What then is the purpose of marriage for Luther you may ask? Luther affirms that it’s simply to satisfy one’s sexual cravings “The body asks for a women and must have it” or again “To marry is a remedy for fornication” – Grisar, “Luther”, vol. iv, pg. 145].
Luther Exposing the Myth says their stated purpose is to show that "from Luther’s own words we shall see him for what he really was, that is a rebellious apostate, who abandoned the faith and led many into apostasy from God under the guise of “reformation” in order to follow his perverse inclinations." With this quote, they attempt to show Luther taught that sex within the confines of marriage is a sinful act.
There are multiple quotes cited by Luther, Exposing the Myth in this snippet. I'm fairly certain all the quotes were taken from Martin Luther, Hitler's Spiritual Ancestor by Peter Wiener. Wiener states,
But, as we have seen before, he has always a very easy way out. It just does not matter whether we commit a sin or not. “You owe nothing to God except faith and confession. In all other things He lets you do whatever you like. You may do as you please, without any danger of conscience whatsoever.” Thus a remedy for his “burning flesh” is easily found. “The sting of flesh may easily be helped so long as girls and women are to be found.” “The body asks for a woman and must have it”; “to marry is a remedy for fornication” (see Grisar, “Luther”, vol. iv, p. 145).
I am reluctant, more than reluctant, to quote some of his sayings; and yet I have to do it if I want to be complete. For the degradation of womanhood and the taking away of all the sacred character of marriage is one of the main reasons why Germany with Luther began its unchristian way down the hill. “Since wedlock and marriage are a worldly business, we clergy and ministers of the Church have nothing to order or decree about it, but must leave each town and country to follow its own usage and custom.” In other words, Luther is not interested in it. Marriage is to him just like any other manual labour, something to be ruled by local traditions, without any kind of Christian standard. “Marriage,” he says, “is an external bodily thing, like any other manipulation.” “Know that marriage is an outward material thing like any other secular business.” “The body has nothing to do with God. In this respect one can never sin against God, but only against one's neighbour”(W12, 131).
But here we come to one of his most contradictory attitudes. For what is usually called “the matrimonial duty”, or “the matrimonial act”, he considers—contrary to the Scripture and Christian ethics—as a great and everlasting sin. The true Christian attitude is best formulated by St. Augustine, who said: “The matrimonial act in order to produce children or to comply with matrimonial duties contains neither guilt nor sin.” This is only logical. For marriage, according to Christian teaching, has been instituted by God in order to propagate humanity, and the commandment of creating children has been given by God—a commandment which cannot be obeyed without a matrimonial act. From this it is quite clear that to obey the will of God can never be a sin in the Christian sense.
Luther is quite opposed to this. “In spite of all the good I say of married life, I will not grant so much to nature as to admit that there is no sin in it . . no conjugal due is ever rendered without sin.” “The matrimonial duty is never performed without sin.” The matrimonial act is, according to Luther, “a sin differing in nothing from adultery and fornication” (W8, 654).
It's likely Wiener didn't actually read Luther in grabbing these quotes, but rather took them from other secondary sources, like Grisar. One thing about Wiener is certain: his method of documentation is haphazard and untrustworthy.
The first quote Luther, Exposing the Myth uses is cited as "Weimar, Vol 8. Pg. 654." WA 8 654 is a page from De votis monasticis Martini Lutheri iudicium (The Judgment of Martin Luther on Monastic Vows). This is not the correct reference for this quote. The quote is actually from WA 10,2, 304 (Uom Eelichen Leben, or The Estate of Marriage). The second quote, "the matrimonial act is 'a sin differing in nothing from adultery and fornication" is though from WA 8 654. The third quote, "The body asks for a women and must have it" is perhaps from WA 10,2 (Uom Eelichen Leben, or The Estate of Marriage) as will be demonstrated below. The fourth quote, "To marry is a remedy for fornication," is directly lifted by Wiener from Grisar:
The increase in the number of matrimonial misunderstandings and quarrels, the haste with which marriage was entered upon and then dissolved, particularly in the Saxon Electorate and at Wittenberg, was not merely the result of the new Evangelical freedom, as Luther and his friends sadly admitted, but was due above all to the altered views on marriage. In the new preaching on marriage the gratification of the sensual impulse was, as will be shown below, placed too much in the foreground, owing partly to the fanatical reaction against clerical celibacy and religious vows. "To marry is a remedy for fornication"; these words of Luther's were again and again repeated by him self and others in one form or another, as though they characterised the main object of marriage [Grisar, Luther IV, p. 145].
Grisar doesn't document the quote which is surprising, since his documentation covers virutally every quote used. Perhaps he only intended it as a summary statement of Luther's view. Such a sentiment is found in Uom Eelichen Leben, and will be documented below. Interestingly, Grisar mis-documented the first quote used by Luther, Exposing the Myth as WA 20, 2, 304. It's actually from WA 10, 2, 304.
Context, Quote One
"In spite of all the good I say of married life, I will not grant so much to nature as to admit that there is no sin in it. .. no conjugal due is ever rendered without sin. The matrimonial duty is never performed without sin."
The first quote comes from The Estate of Marriage (1522). Luther is commenting on those who mistakenly chose a life of celibacy, and concludes the entire treatise by stating:
To sum the matter up: whoever finds himself unsuited to the celibate life should see to it right away that he has something to do and to work at; then let him strike out in God’s name and get married. A young man should marry at the age of twenty at the latest, a young woman at fifteen to eighteen; that’s when they are still in good health and best suited for marriage. Let God worry about how they and their children are to be fed. God makes children; he will surely also feed them. Should he fail to exalt you and them here on earth, then take satisfaction in the fact that he has granted you a Christian marriage, and know that he will exalt you there; and be thankful to him for his gifts and favors.
With all this extolling of married life, however, I have not meant to ascribe to nature a condition of sinlessness. On the contrary, I say that flesh and blood, corrupted through Adam, is conceived and born in sin, as Psalm 51[:5] says. Intercourse is never without sin; but God excuses it by his grace because the estate of marriage is his work, and he preserves in and through the sin all that good which he has implanted and blessed in marriage [LW 45:48].
Luther had exalted the state of marriage throughout this treatise with statements like, "we may learn how honorable a thing it is to live in that estate which God has ordained. In it we find God’s word and good pleasure, by which all the works, conduct, and sufferings of that estate become holy, godly, and precious..."[LW 45:41]. Here at the end, Luther is carefully pointing out that sin is not absent from marriage, and that every aspect of human existence is tainted with sin. Sin is transferred through the means of intercourse. All children are born with a sin nature inherited from their parents. This is enough to explain why intercourse is never without sin. Luther though explains elsewhere,
If Adam had not fallen, the love of bride and groom would have been the loveliest thing. Now this love is not pure either, for admittedly a married partner desires to have the other, yet each seeks to satisfy his desire with the other, and it is this desire which corrupts this kind of love. Therefore, the married state is now no longer pure and free from sin [LW 44:8].
Context, Quote Two
...The matrimonial act is “a sin differing in nothing from adultery and fornication.”
The second quote comes from The Judgment of Martin Luther on Monastic Vows. Luther is in the midst of explaining those who have taken a vow of chastity but find they can't keep the vow should seek to be married: "it is my belief that if anyone is unable to keep his vow of chastity and takes a wife, confident of God’s mercy, as he grows in this faith he will discover a merciful and understanding Father. After all, he is doing this to avoid sinning more grievously against God’s law" [LW 44:376]. He then states,
In no sense does God attribute sin to the conjugal rights of married people, which is due solely to his mercy, although Psalm 51[:5] refers to it as sin and iniquity in no way differing from adultery and whoredom, because it springs from passion and impure lust. It is impossible to avoid this emotion, since we are restrained to forego it. Why then should it not also be supposed in the case of a monk who is unable to keep his vow of chastity, and who would otherwise sin, that this impossible vow may be relaxed, and that once the vow is nullified, he be permitted to marry? Or, if it is done in sin while still under the vow, why may it not be mercifully pardoned as is a conjugal right? Consider the immensity of the law God gave us, and then compare the following points carefully. Consider the impossibility of keeping the law, our sin which is forgiven, and the boundlessness of his mercy and goodness. And then, compare all that with the foolishness and stupidity of our vow, a mere tradition and invention of men. The reasonableness of this argument will most certainly compel you to consider it a small thing for the marriage of a stumbling celibate who is unable to keep his vow of chastity to be excused, compared with the forgiveness of sin in the law of God, because no man can keep that law. This law is valid for us all. Consequently, from this work of God’s mercy we may conclude with absolute confidence that an impossible vow is not binding as far as the goodness of God is concerned, even though it may be binding in some way within its own limitations [LW 44:376].
Notice Luther begins by explicitly stating "In no sense does God attribute sin to the conjugal rights of married people." That's far different than "for Luther the matrimonial act is 'a sin differing in nothing from adultery and fornication'" as stated by Luther, Exposing the Myth. Luther then states, "although Psalm 51[:5] refers to it as sin and iniquity in no way differing from adultery and whoredom, because it springs from passion and impure lust. It is impossible to avoid this emotion, since we are restrained to forego it." He appears to interpret David's statement "Behold, I was brought forth in iniquity, And in sin my mother conceived me" to mean that even marital sex has passion and impure lust involved. Luther isn't saying that marital intercourse is of the same nature as adultery and whoredom, but that it shares a similar drive. In his commentary on Psalm 51 he states that David "is not talking about sin in marriage or about the sin of parents; as though he were accusing his parents of sin" [LW 12:347].
Context, Quote Three
The third quote, "The body asks for a women and must have it" is perhaps from The Estate of Marriage. At the very beginning of this treatise, Luther states:
In the first part we shall consider which persons may enter into marriage with one another. In order to proceed aright let us direct our attention to Genesis 1[:27], “So God created man … male and female he created them.” From this passage we may be assured that God divided mankind into two classes, namely, male and female, or a he and a she. This was so pleasing to him that he himself called it a good creation [Gen. 1:31]. Therefore, each one of us must have the kind of body God has created for us. I cannot make myself a woman, nor can you make yourself a man; we do not have that power. But we are exactly as he created us: I a man and you a woman. Moreover, he wills to have his excellent handiwork honored as his divine creation, and not despised. The man is not to despise or scoff at the woman or her body, nor the woman the man. But each should honor the other’s image and body as a divine and good creation that is well-pleasing unto God himself.
In the second place, after God had made man and woman he blessed them and said to them, “Be fruitful and multiply” [Gen. 1:28]. From this passage we may be assured that man and woman should and must come together in order to multiply. Now this [ordinance] is just as inflexible as the first, and no more to be despised and made fun of than the other, since God gives it his blessing and does something over and above the act of creation. Hence, as it is not within my power not to be a man, so it is not my prerogative to be without a woman. Again, as it is not in your power not to be a woman, so it is not your prerogative to be without a man. For it is not a matter of free choice or decision but a natural and necessary thing, that whatever is a man must have a woman and whatever is a woman must have a man [LW 45:17].
The quote could also be from Luther'sletter of March 27, 1525 to Wolfgang Reissenbusch. He states,
I believe, honored sir, that you are convinced of what I say, and that you are not troubled by such scruples, but I fancy that human fear and timidity lie in your way, as it is said that he must be a bold man who dares to take a wife. There is then the more need to encourage, counsel and urge you, making you eager and bold. Dear and honored sir, why should you torture yourself and strive with your own thought? It cannot be otherwise than that you think of these things. Thoughts come from the sense and are right merry. Your body urges you to marry and needs it; God wills and forces it What will you do about it?
Even if these aren't the context from which Wiener took his quote, they certainly express Luther view. Luther, Exposing the Myth states the purpose of marriage for Luther is simply to satisfy one’s sexual cravings. In this context, the point is simply about the nature of human beings. Elsewhere Luther gives a definition of marriage:
This is a true definition of marriage: Marriage is the God-appointed and legitimate union of man and woman in the hope of having children or at least for the purpose of avoiding fornication and sin and living to the glory of God. The ultimate purpose is to obey God, to find aid and counsel against sin; to call upon God; to seek love, and to educate children for the glory of God; to live with one's wife in the fear of God and to bear the cross" [WA 43, 310].
Context, Quote 4
"To marry is a remedy for fornication" may actually not have a context. The quote comes from Grisar, and it appears he's making a summary statement of Luther's view. In The Estate of Marriage, Luther does state:
Observe that thus far I have told you nothing of the estate of marriage except that which the world and reason in their blindness shrink from and sneer at as a mean, unhappy, troublesome mode of life. We have seen how all these shortcomings in fact comprise noble virtues and true delight if one but looks at God’s word and will, and thereby recognizes its true nature. I will not mention the other advantages and delights implicit in a marriage that goes well—that husband and wife cherish one another, become one, serve one another, and other attendant blessings—lest somebody shut me up by saying that I am speaking about something I have not experienced, and that there is more gall than honey in marriage. I base my remarks on Scripture, which to me is surer than all experience and cannot lie to me. He who finds still other good things in marriage profits all the more, and should give thanks to God. Whatever God calls good must of necessity always be good, unless men do not recognize it or perversely misuse it.
I therefore pass over the good or evil which experience offers, and confine myself to such good as Scripture and truth ascribe to marriage. It is no slight boon that in wedlock fornication and unchastity are checked and eliminated. This in itself is so great a good that it alone should be enough to induce men to marry forthwith, and for many reasons.
The first reason is that fornication destroys not only the soul but also body, property, honor, and family as well. For we see how a licentious and wicked life not only brings great disgrace but is also a spendthrift life, more costly than wedlock, and that illicit partners necessarily occasion greater suffering for one another than do married folk. Beyond that it consumes the body, corrupts flesh and blood, nature, and physical constitution. Through such a variety of evil consequences God takes a rigid position, as though he would actually drive people away from fornication and into marriage. However, few are thereby convinced or converted [LW 45:43].
Once again, a context makes quite a difference. Luther was not an enemy of marriage, but spoke highly of it, and valued it. Such sentiment can be found throughout his writings. This type of information though was ignored by Luther, Exposing the Myth. It's also very important to keep in mind the historical climate from which Luther's remarks germinated. In the article The Reformation of Marriage Law in Martin Luther's Germany: Its Significance Then and Now, John Witte notes the following:
Three broad perspectives on marriage are found in the Roman Catholic tradition of the late eleventh through fifteenth centuries. Marriage was viewed (1) as a created, natural institution, subject to the laws of nature; (2) as a sacrament of faith, subject to the laws of Scripture; and (3) as a contract, subject to the general canon laws of contract formation, maintenance, and dissolution. These three perspectives were, in an important sense, complementary, each emphasizing one aspect of marriage: its divine origin, its symbolic function, and its legal form respectively. There was, nevertheless, a certain tension among these three perspectives as well, which manifested itself in the laws of marriage.
Marriage was regarded, first, as a created natural institution which serves both as "a duty for the sound and a remedy for the sick." Already in Paradise, God had commanded man and woman to "be fruitful and multiply." He had created man and woman as social beings, naturally inclined to one another. He had endowed them with the physical capacity to join together and beget children. He had commanded the man and the woman to help and nurture each other and to inculcate within their children the highest virtue and love of the Divine. These qualities and duties continued after the Fall into sin. But after the Fall, marriage also came to serve as a remedy for the individual sinner to allay his lustful passion, to heal his incontinence, and to substitute a bodily union with a spouse for the lost spiritual union with the Father in Paradise. Rather than allow sinful people to burn with lust, God provided the institution of marriage wherein people could direct their natural drives and desires toward the service of the human community.
Many theologians and canonists, however, subordinated the duty of propagation to that of celibate contemplation, the natural drive for sexual union to the spiritual drive for communion with the God.For, as Peter Lombard put it,
"The first institution [of marriage in Paradise] was commanded, the second permitted... to the human race for the purpose of preventing fornication. But this permission, because it does not select better things, is a remedy not a reward; if anyone rejects it, he will deserve judgment of death. An act which is allowed by permission, however, is voluntary, not necessary."
After the Fall, marriage remains a duty, but only for those tempted by sexual sin. For those not so tempted, marriage is only an inferior option. It is far better and far more virtuous to remain celibate and to contemplate. For marriage is an institution of the natural sphere, not the supernatural sphere. Though ordained by God and good, it serves primarily for the perfection of the human community not for the perfection of the individual. Participation in it merely keeps man free from sin and vice. It does not directly contribute to his virtue. The celibate, contemplative life, by contrast, is a calling of the supernatural sphere. Participation in it increases man's virtue and aids him in the pursuit of beatitude. To this pursuit, "marriage is a very great obstacle," for it forces man to dwell on the carnal and natural rather than the spiritual and supernatural aspects of life.
While retaining certain aspects of this, Luther's views on marriage challenged this popular sentiment:
Unlike many Roman Catholics, however, the reformers taught that all persons should heed the duty and accept the gift of marriage. By stressing God's moral and pedagogical functions for the family in society, alongside its procreational function, and by defining for the family its own created sphere of authority and responsibility, alongside that of the church and the state, the reformers accorded great importance to the institution. The married couple, the family, was seen as an important, independent institution of creation. It was as indispensable an agent in God's redemption plan as the church had been for the Roman Catholics. It, too, was in Luther's words, "a divine and holy estate of life," a "blessed holy calling," an institution with created social tasks. The family was to teach all persons, particularly children, Christian values, morals, and mores. It was to exemplify for a sinful society a community of love and cooperation, meditation and discussion, song and prayer. It was to hold out for the church and the state an example of firm but benign parental discipline, rule, and authority. It was to take in and care for wayfarers, widows, and destitute persons a responsibility previously assumed largely by monasteries and cloisters. The family thus no longer stood beneath the church but alongside it. The tasks to which its members were called were as vital and virtuous as the tasks of the church officials. Marriage was thus not to be viewed as an inferior option, but rather as a divine calling and a social status desirable for all people.
I highly reccomend this entire article, found here.