Augustine of Hippo, Against Two Letters of the Pelagians I.33: The Shame of Nakedness

This kind of shame—this necessity of blushing—is certainly born with every man, and in some measure is commanded by the very laws of nature; so that, in this matter, even virtuous married people are ashamed. Nor can any one go to such an extreme of evil and disgrace, as, because he knows God to be the author of nature and the ordainer of marriage, to have intercourse even with his wife in any one’s sight, or not to blush at those impulses and seek secrecy, where he can shun the sight not only of strangers, but even of all his own relatives. Therefore let human nature be permitted to acknowledge the evil that happens to it by its own fault, lest it should be compelled either not to blush at its own impulses, which is most shameless, or else to blush at the work of its Creator, which is most ungrateful. Of this evil, nevertheless, virtuous marriage makes good use for the sake of the benefit of the begetting of children. But to consent to lust for the sake of carnal pleasure alone is sin, although it may be conceded to married people with permission.