No doubt Paul was unaware of the distinction between sexual orientation, over which one has apparently very little choice, and sexual behavior, over which one does. He seemed to assume that those whom he condemns are heterosexual, and are acting contrary to nature, "leaving," "giving up," or "exchanging" their regular sexual orientation for that which is foreign to them. Paul knew nothing of the modern psychological understanding of homosexuals as person whose orientation is fixed early in life, persons for whom having heterosexual relations would be contrary to nature, "leaving," "giving up" or "exchanging" their natural sexual orientation for one that was unnatural to them.
In other words, Paul really thought that those whose behavior he condemned were "straight," and that they were behaving in ways that were unnatural to them. Paul believed that everyone was "straight." He had no concept of homosexual orientation. The idea was not available in his world. there are people who are genuinely homosexual by nature (whether genetically or as a result of upbringing no one really knows, and it is irrelevant). For such a person it would be acting contrary to nature to have sexual relations with a person of the opposite sex.

I'm afraid Blacknad, here I must disagree with you. Its like saying that stealing a pin is the same as stealing 1 million dollars. When the law states, THOU SHALT NOT STEAL. Homosexuality and lesbianism must be categorised where they belong. I personally also had a friend who was gay and effeminate, so I know what I am talking about. Although I do not like to judge, for the Lord warned us sternly about judging others. But for my own part I feel that such acts are demonic influences. You have no idea how clever and subtle these demons are. They will make thoughts appear in your mind, and even make one sick. Remember the demoniac child in Mark. Therefore such acts must be labelled what they are and that is EVIL! No amount of fiddling with the scriptures can justify them. If animals commit such acts, remember, they do not have a free will like us and nor do they have a consience. Try stepping into a cage of lions and see if they reason with you. Also who is to deny that these animals are also under demonic influence in order to influence us into accepting such behavior. The greatest achievement of Satan today is that he has convinced mankind that he and his demons do not exist. Well I can categorically say that they do exist. Read the story of when they wanted to film 'The Omen' and how they were beset by mishaps, and you will find that, that particular movie caused many people to read the bible. Bible sales for the year this film came out were record breaking and still are. So you can now see why Satan and his demons tried to no avail to prevent this film from being made.