Here Paul is using language frequently used in antiquity which knew no such terms as homosexual or lesbian, to refer to homosexual (unnatural) and heterosexual (natural) sexual behavior. One can not retort that Paul, no biologist, failed to realize that to some persons it seems "natural" to be attracted to those of the same gender. Paul is well aware of how sin becomes ingrained in human nature, to such an extent that it comes to seem perfectly natural. No, Paul is contrasting the creation order as originally given, versus various manifestations of human fallenness.
For any who might dismiss Paul as just another homophobic early Jew picking on an oppressed minority, Paul says at the end of Romans 1 that sinfulness that God condemns is also manifested by covetousness, malice, envy, murder, slander, insolence, rebellion against parents, ruthlessness, deceit, pride and the like. He is then able to say in Romans 2:1 "Therefore you have no excuse, whoever you are, when you judge others; for in passing judgment on another you condemn yourself, because you that judge are doing the very same sort of sinful things."
In other words, all of us have fallen and can't get up without the grace of God. This means there must be no singling out of homosexual sin as somehow more heinous sin than the sins heterosexuals commit. Homophobia or heterosexual sexual activity outside marriage is a sin just as surely as same-sex sexual activity is.
We must turn now to two other Pauline texts. The first is 1 Corinthians 6:9-10. Here Paul is trotting out a vice list of certain kinds of behaviors which will keep a person out of the final Kingdom of God. It is a general list that does not single out homosexual behavior but includes it along with fornicators, idolators, adulterers, thieves, the greedy, drunkards, revilers and robbers.
The two terms in question are the Greek malakoi and arsenokoitai. The former term literally means something like effeminate or soft, and refers to the passive partner in a homosexual tryst. The other term is not found in Greek literature prior to Paul, and may even have been coined by Paul. It means literally a male copulator, someone who copulates with a male. Paul not only reminds his largely gentile Christian audience that some of them had practiced one or another of these vices, but points out that some seemed to be still persisting in them.
The witness of the Bible is univocal about same sex sexual activity. It is always rejected as sinful. There is no distinction made between homosexual behavior that is part of the consensual acts of adults and other forms of such behavior. Indeed most ancients were more likely to condemn consensual adult homosexual behavior than pederasty-- just the opposite of the situation today. We see such a condemnation in antiquity as early as Plato and as late as Josephus and beyond.
For those who indeed intend to continue to recognize the Bible as the normative rule, not only for faith but for ethical practice in the church or the synagogue, we cannot be about the business of making anyone comfortable in their sin, whether a heterosexual or a homosexual person. Equally, however, the way of Jesus in treating such situations is that balance of justice and mercy that says on the one hand "Neither do I condemn you" and on the other "Go and sin no more."