There's a new book out called From Shame to Sin: The Christian Transformation of Sexual Morality in Late Antiquity and a really good review in the NY Review of Books is helpful for putting some of this in context. The book (and review) highlights the transition from one culture (the Romans) to another (the Christians) that was very different.
The Romans were busy having lots of sex, but it was intertwined with prostitution and especially, slavery. According to the review and the book, slavery looms above it all. yet it was the sex that got the attention.
The reviewer explains
From Saint Paul onward, the great issues of sex and freedom were brought together in Christian circles like the enriched ore of an atomic device. For Paul, porneia—fornication—meant a lot more than premarital fooling around. It was a brooding metonym, “enriched” by an entire spectrum of associations. It stood for mankind’s rebellion against God. And this primal rebellion was shown most clearly in the topsy-turvy sexual freedom ascribed first by Jews and then by Christians to the non-Christian world.So is all the strum und drang of modern Christianity and the historical opposition to gay people, still a reaction to the sexual orgies of the Romans and their slaves? and it was the sex more than the slavery that got their attention?
But then, what was true freedom? Freedom also was a mighty metonym, of which the freedom to decide one’s sexual fate was only one, highly “enriched” part. Above all, it meant “freedom” from “the world.” And by “the world” Christians meant, bluntly, the Roman society of their own times, where unfreedom was shown in its darkest light by the trading and sexual abuse of unfree bodies. It no longer mattered, to Christians, with whose bodies, from which social categories, and in what manner sex might happen. From Paul onward, for Christians, there was right sex—sex between spouses for the production of children; wrong sex—sex outside marriage; and abhorrent sex—sex between same-sex partners. Wrong sex of any kind was a sin. And a sin was a sin
In any case, the book sounds fascinating.