Actually, homosexuality has been prevalant throughout most of human history and through different cultures. It wasn't until the adoption of Christianity by the Romans that laws restricting the practice came into existance. In fact, same sex marriage had existed in Rome prior to the adoptoin of Christianity and specific laws had to be created to ban it. Most of the laws across the world against homosexuality were largely a result of Western influence through colonialism. As far as the "laws of nature" homoexuality is observable in hundreds of species of animals, but I'm not one to get my morals based on what is natural.
This is not completely accurate. Homosexuality was publicly frowned upon by ancient Romans, though so-called "Greek Love" was quite popular and normal...in GREECE. Paul himself was a Roman citizen, and I actually think he may have been affected by the prevailing viewpoint in Roman society.
Being labeled as gay would end a man's role in society and/or politics. Most Roman insults have to do with homosexual sex acts...calling someone a fellator (********er) was an unforgivable slur. Any man who was tainted by association with homosexuality, particularly in Gaius Julius Caesar's era, or before, would have been irreparably tarnished and would have difficulties in society.
So...did Romans practice homosexuality? Yes. But, it was practiced covertly and there were many prejudices against gays in ancient Rome.
As far as Paul's writings on sexual immorality...Christianity had many Jewish sensibilities, and the Jewish Christians expected non-Jewish converts to honor them. For many years, until the destruction of Jerusalem 30+ years after the death of Christ, Jerusalem was the seat of Christianity, and Christianity was a Jewish sect. At the same time, Paul was eagerly seeding Christian churches in areas that were heavily Greek, and in these communities, there was open male/female prostitution, both in the community and in the pagan temples.
You can see the extreme tension between the Jewish and the Gentile converts to the faith in much of the new testament, particularly Acts and Romans.
That's part of the reason for Paul's proscriptions on sexual immorality, because those sexual practices were normal in the cities of Ephesus, Thessalonica, etc., and the Jewish converts to Christianity found these practices appalling in the extreme. Paul was trying to bring together a single entity from wildly disparate cultures...
The other part was the Christianity was perceived, in Rome, to be just another one of the far eastern mystical religions, and those faiths had all kinds of bizarre practices. Within the cult of Isis, for instance, males would frequently, and publicly, genitally mutilate themselves as a form of worship. Paul was not only trying to set a pattern for Christian behavior, but he was also trying to distinguish Christianity from other, competing forms of mystical faith.
And the Romans already believed weird things about the Christians...accusing them of odd sexual practices and cannibalism.
Once the seat of power in the Christian church shifted to Rome, after the fall of Jerusalem and the destruction of the temple (which scattered the Christians throughout the Mediterranean), cleaning up these misperceptions was a major concern.