Why are you so certain that homosexuality is sin? What principles do you follow when reading scripture and interpreting its messages?
We must be aware of quite a few things there. For example, the distinction between law and gospel, the fact that Jesus brought grace and no new laws, that belief in Jesus is the end of the law, that even in New Testament times themselves the new christians abolished the use of some of the mosaic code, that Paul was not perfect and himself spoke of looking into a glass darkly, that he wanted us to get forward in the faith and that things can change.
Just for example, take slavery. Paul didn't attack slavery and actually told slaves to bear with their lot and to serve their masters willingly. But in our day we decided that slavery is bad, we call it a sin.
In my opinion, we cannot define sin as the transgression of the mosaic law anymore, instead it is the privation of the good. The law was ONCE used as a means to find out sin, but Paul argued it would only produce sin because sin uses god's commandments to kill us. So we really don't have commandments anymore but instead God draws, calls us, tells us things and warns us of spiritual death and the consequences of it that stem from living contrarily to him.
And historically, the church sat down and examined the law and the morality of the time and it picked up what was good and rejected what was not so good. So for example, they abolished stoning because of Christ's example.
And Paul clearly wrote against what happened in Rome. Look at the prophetic books of the Old Testament, it contains no complaint of God about homosexuality. It was a certain purity law given TO THE JEWS, it did not for all time say that homosexuality is truly unwanted by God. Furthermore Paul writes about men and woman EXCHANGING the use of the other sex in sexuality, meaning they speak about people who were embracing heterosexuality but then, on spurious reasons, decided to sleep with people of the same sex. He doesn't write about the ordinary gay person who pretty much never felt any sexual attraction to the opposite sex. Do you see where this is going? It's coming down to adultery again, it's about keeping marriage intact and not spoiling it with the lust for sexual adventures.
And look a bit at the times ... just read up on roman and greek decadence and you will clearly see that Paul speaks about an IMMORAL sexuality, including immoral homosexuality, not about homosexuality at large because that would have been incoherent with our being set free of the law as jews or our not being inducted into the law obedience as Gentiles.