When it says "married but once;" this should be, understood to mean that someone who is going to be ordained should neither have been married to a widow nor have been married twice. This impediment to holy orders is not produced by any kind of sin but rather, so to speak, through a defect in the sacrament. For matrimony is a sacrament of the body, that is, a sign of the marriage between Christ and the Church. Thus, just as Christ married only one virgin church, a priest, who is Christ's vicar, should have been the husband of only one woman who was a virgin. The church regards a virgin as any woman who has not been previously married, even though she may have lost her virginity. Thus anyone who marries a widow is not able to be raised to the priesthood. If he married a woman who had already lost her virginity, he can be ordained.
Saints Jerome and Augustine disagreed over this point. Jerome argued that if someone married one woman before he was baptized and then another after his baptism, he should not be considered to be twice married since baptism removed the impediment that had arisen from the first marriage. But Augustine's position prevailed, which argued that baptism removed nothing but sin and its consequences. Marriage, however, was neither a sin nor were its consequences sinful. Thus, the first marriage and its consequences were not removed by baptism.
When some other canonical impediment has occurred due to vice—for instance if someone has been convicted of theft or adultery—that infamy can be removed by baptism. But if someone has committed murder and thus been-disgraced, this disqualification is not removed even after baptism since it is such a horrible thing to shed blood.
Source: John Shinners and William Dohar, Pastors and the Care of Souls in Medieval England (Indiana: University of Notre Dame Press, 1998) 7-8.
If Thomas of Chobham summed up the views of Augustine and Jerome correctly, I think it demonstrates that simply because Augustine and Jerome are considered important church fathers, this doesn't mean that if you sift back into church history, everyone was saying the exact same thing all the time. It also shows that simply because both of these great men were closer in time to the Apostles this does not necessarily mean they have a better perspective than someone living today does. Third, I think both of them actually don't interpret the verse with any sort exegetical justice. Fourth, when I Googled around to see what some of Rome's infallible interpreter apologists were saying on 1 Tim. 3:2, I ended up with things like this.