The following is from the web page Luther, Exposing the Myth, under the heading "Marriage and Women":
"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" [Weimar, Vol. 12, Pg. 131].
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 Christ taught the value of marriage as spiritual, while Luther taught marriage was secular, and in it, one can't sin against God.
Luther, Exposing the Myth probably took this quote from Martin Luther, Hitler's spiritual Ancestor by Peter Wiener. Wiener states,
I am reluctant, more than reluctant, to quote some of [Luther's] 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 hi..[sic] “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).
It's likely Wiener didn't actually read Luther in grabbing these quotes, but rather took them from other secondary sources. With the quote in question, I'm certain he didn't get it from the source he (and Luther Exposing the Myth) cite. Here is Wiemar Vol. 12 page 131. While the immediate context does include some passing comments on marriage, there is nothing even remotely similar to this quote in question on the page. WA 12, 131 is a page from Luther's commentary on 1 Corinthians 7. The English translation is found in LW 28 (WA 12, 131 can be found in LW 28:44-45). The reference is bogus.
Notice the quote, as cited by Luther, Exposing the Myth, doesn't make much sense? If one looks closely, it appears Wiener is using two different quotes, which Luther, Exposing the Myth didn't catch. The first part of the quote comes from WA 10,2, 283, "Uom Eelichen Leben," or The Estate of Marriage, which is found in LW 45:25. The other part of the quote probably comes from WA 12, 132 (which is found in LW 28:46).
The Estate of Marriage discusses the eighteen impediments of marriage which ruled the day during Luther's time. These contained rules on who was allowed to marry who. Luther was discussing the fifth impediment, that one is not allowed to marry "a Turk, a Jew, or a heretic"[LW 45:24]. Luther says such a rule is in direct contradiction to 1 Corinthians 7:12-13 (if any brother has a wife who is an unbeliever, and she consents to live with him, he must not divorce her. And a woman who has an unbelieving husband, and he consents to live with her, she must not send her husband away). Luther then states,
Know therefore that marriage is an outward, bodily thing, like any other worldly undertaking. Just as I may eat, drink, sleep, walk, ride with, buy from, speak to, and deal with a heathen, Jew, Turk, or heretic, so I may also marry and continue in wedlock with him. Pay no attention to the precepts of those fools who forbid it. You will find plenty of Christians—and indeed the greater part of them—who are worse in their secret unbelief than any Jew, heathen, Turk, or heretic. A heathen is just as much a man or a woman—God’s good creation—as St. Peter, St. Paul, and St. Lucy, not to speak of a slack and spurious Christian [LW 45:25].
That's the first line of the quote in question. The context is about the freedom to marry whom one will, which should not be regulated by the church.
The second part of the quote comes from LW 28. Luther while commenting on 1 Cor. 7:23, addresses this same theme: "When someone teaches that a Christian may not marry a non-Christian and remain with her—as the rules of the church do—he obstructs the freedom that St. Paul teaches us here and forces people to obey these rules more than God’s Word" [LW 28:44]. He wraps up that section by noting while God has given us freedom from such rules, the rules of particular relationships among people are still intact. For instance, wives are still subject to their husbands.
Luther then begins commenting on 1.Cor. 7:24, building upon this idea of responsibility in human relationships. God has set his people free from keeping the law in order to achieve salvation. The only responsibility of a Christian is to believe and confess (LW 28:45). On the other hand, Christians are still responsible to their neighbors:
But because in this relationship you are bound up with your neighbor and have become his servant, it is God’s will that no one be deprived of what is his by means of His freedom but rather that those things of your neighbor be protected. For although God pays no attention to these things on His own account, He pays attention to them on account of your neighbor. This is what he means with the words “with God,” as though He were admonishing us: “I did not make you free among men or with your neighbor, for I do not wish that which is his taken from him until he gives you permission. But you are entirely free with Me and cannot ruin yourself in My sight by keeping to or refraining from outward things.” Therefore notice this and differentiate between the freedom existing in your relation to God and the freedom existing in your relation to your neighbor. In the former this freedom is present, in the latter it is not, and for this reason: God gives you this freedom only in the things that are yours, not in what is your neighbor’s. There differentiate between what is yours and what is your neighbors. That is why no man can leave his wife, for his body is not his own but his wife’s, and vice versa. Likewise the servant and his body do not belong to him himself but to his master. It would be of no importance to God if the husband were to leave his wife, for the body is not bound to God but made free by Him for all outward things and is only God’s by virtue of inward faith. But among men these promises are to be kept. In sum: We owe nobody anything but to love (Rom. 13:8) and to serve our neighbor through love. Where love is present, there it is accomplished that no eating, drinking, clothing, or living in a particular way endangers the conscience or is a sin before God, except when it is detrimental to one’s neighbor. In such things one cannot sin against God but only against one’s neighbor [LW 28:45].
In this context, Wiener's "the body has nothing to with God" simply means that a Christian has been given freedom by God. There are no rules to be kept to keep one justified before him. On the other hand, the body could still very well belong to another, like a husband's body is the property of his wife. Wiener's "one can never sin against God, but only against one's neighbour" simply means that if one is judged by God covered with the righteousness of Christ, sin against God is not counted. On the other hand, a sin against one's neighbor is still indeed a sin against one's neighbor.
Luther taught a life under the cross, which is a life of discipleship of following after Christ. Our crosses though, do not save. They serve the neighbor. We are called to be neighbor to those around us. Justification is by faith alone unto good works done for the good of one’s neighbor. This idea runs throughout Luther's written corpus.
For Luther, marriage was indeed "the God-appointed and legitimate union of a man and woman" with its ultimate purpose to glorify God (WA 43:310). But as to its working and maintenance, Luther sought to have such matters governed by the state. It wasn't the job of the church to come up with marriage rules and regulations. Like clothing, food, and houses, the working of marriage should be something regulated by secular authority.
While some may think such a paradigm sets forth humanistic anarchy, keep in mind in Luther's time the power of the Roman church had perpetuated a mess with marriage regulations, and his was a direct reaction to that. The worldview at the time took for granted that marriage was between a man and a woman. He did not foresee a secular authority that would redefine marriage (i.e. homosexual marriage). Of such a legalization, he would have been horrified ["God revenges and punishes the forbidden marriage, so that Sodom and Gomorrah, which God overwhelmed in days of old with fire and brimstone" LW 46:198]. Nor would he be against rebuking a modern-day Pilate (WA 28:361), and that passive Resistance in certain conditions is justified.
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.