Consequently Paul's unambiguous condemnation of homosexual behavior in Roman 1:26 - 27 must be the centerpiece of any discussion.
"For this reason God gave them up to dishonorable passions. Their woman exchanged natural relations for unnatural, and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in their own persons the due penalty for their error."
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.
Likewise the relationships Paul describes are heavy with lust; they are not relationships of consenting adults who are committed to each other as faithfully and with as much integrity as any heterosexual couple. That was something Paul simply could not envision. Some people assume today that venereal disease and AIDS are divine punishment for homosexual behavior; we know it as a risk involved in promiscuity of every stripe, homosexual and heterosexual. In fact, the vast majority of people with AIDS around the world are heterosexuals. We can scarcely label AIDS a divine punishment, since non-promiscuous lesbians are at almost no risk.
And Paul believes that homosexuality is contrary to nature, whereas we have learned that it is manifested by a wide variety of species, especially (but not solely) under the pressure of overpopulation. It would appear then to be a quite natural mechanism for preserving species. We cannot, of course, decide human ethical conduct solely on the basis of animal behavior or the human sciences, but Paul here is arguing from nature, as he himself says, and new knowledge of what is "natural" is therefore relevant to the case.