I had always assumed that the early Christian anti-sex agenda was thought up by St Paul, whose role in the formation of the early church was so pervasive. But a close reading of Acts 15:1-35 puts the matter in a very different perspective, and apparently provides the oldest record pertaining to sexual prohibitions in the history of Christianity.
Around 20 years after Jesus' death, Paul travels to Jerusalem to discuss with the local elders what position should be taken toward the Gentiles -- should the church admit them, or should it remain a sect of Judaism?
The Council of Jerusalem, as Biblical editors later called it, comes to a rather remarkable decision: Gentiles are invited to join, and male members do not have to be circumcised. (This is clearly a water-shed moment; the new church will not remain a sect within an ethnic-based religion, but will become so to speak an international movement -- and you get to hold on to your foreskin.)
But there are two conditions attached: new converts must adhere to traditional Jewish dietary laws, and they must agree to "abandon fornication," as the older English versions put it, or give up sexual immorality, as newer translations suggest. (The operative word in Greek is pornea, which in our present context may have simply meant "sex," devalorized.)
The Council of Jersualem's decision suggests a compromise solution reached between opposing factions: the church will expand and go public, but only if the Gentiles agree to eat kosher foods, and also to follow traditional Jewish restrictions regarding sexual practices.
Why is it important? Because the decision determined the turning point when European civilization abandoned its long history of comparatively amoral, free-range sexuality, and when the body became not only an object of moral urgency, but also a hostage to religious authority.