As the British philosopher Elizabeth Anscombe noted, in this Christianity was a great equalizing force: because of the fact of pregnancy, most pre-modern cultures enforce sexual restraint on women. Where Christianity’s bizarreness lay is that it insisted on the same restraint on the part of men — whether gay or straight. Christians held a bizarrely exalted view of (lifelong, monogamous, fertile, heterosexual) marriage as reflecting the image of God himself, but, even more bizarrely, held up lifelong celibacy as an even more exalted state of life. From the start, alongside the refusal to worship the Roman emperor as a god and Christians’ supererogatory care for the poor, this was what set Christians apart, and goes a long way toward explaining why Pagan writers could scorn Christianity as a religion of “slaves and women.”
Of course, like all ideals, this was very often observed in the breach, but such is the lot of human nature. Human beings, societies, cultures, and religions have a worldview which include moral “oughts,” and which they only partially live up to, as anyone who has tried to stick to a diet knows.
But the point is clear: From the start, Christians embodied a different way of life. From the start, they understood a particular sexual ethic to be a keystone of this way of life. And they understood the logic of this ethic as prohibiting (among other things) homosexual acts.