For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth.  For what can be known about God is plain to them, because God has shown it to them.  For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse.  For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened.  Claiming to be wise, they became fools,  and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things.Now that we live in an enlightened age in which we are no longer bound by the constraints of tradition or the ordinary meaning of words, it has become fashionable to describe the nature of which Paul speaks as a personal or individual nature and not a state of creation or all humanity. So when Paul writes of “unnatural” same-sex sexual relations, he cannot possibly be describing the intimacy of those attracted to the same sex. These folks are naturally oriented in this way (born not made) and therefore what is natural to them may only appear to be unnatural to the rest of us.
 Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves,  because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen.
 For this reason God gave them up to dishonorable passions. For their women exchanged natural relations for those that are contrary to nature;  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 themselves the due penalty for their error.
 And since they did not see fit to acknowledge God, God gave them up to a debased mind to do what ought not to be done.  They were filled with all manner of unrighteousness, evil, covetousness, malice. They are full of envy, murder, strife, deceit, maliciousness. They are gossips,  slanderers, haters of God, insolent, haughty, boastful, inventors of evil, disobedient to parents,  foolish, faithless, heartless, ruthless.  Though they know God's righteous decree that those who practice such things deserve to die, they not only do them but give approval to those who practice them. (Romans 1:18-32 ESV)
For those who identify as gay, same-sex intimacy is not “unnatural” -- just the opposite -- relations with the opposite sex are unnatural. So, follow the logic, Paul is here condemning heterosexual people who “unnaturally” depart from their own kind of sex and homosexual people who do the same. Like Shakespeare, Paul has boiled sexual morality down to "to thine own self be true." Sex for Paul is not a frontier to be explored but an identity to which you must be true -- as God made you. Since sexual orientation is unchosen and virtually permanent, same-sex sexual expression is “natural” to gay and "unnatural" to straight. So Paul cannot possible be requiring gay people to offend their nature any more than he is requiring straight people to offend their nature.
Ahhhh. Where to begin? First of all is a logistical problem. If you are bi, none of this must apply to you since your nature is flexible (love the one you're with). If there was ever an advantage to being bi, it is surely this; you are exempt from Paul's condemnations and free to indulge, experiment, and experience whatever feels good. Because that is exactly what Paul would say, right?
Then there is the problem of "nature." Is Paul referring to some individual nature or to nature in the sense of God's creative will and order? Such is the progress of morality, sexuality, and Scripture in pursuit of a Biblical ethic that will sanction same-sex relations and relationships. Nature is not the creature of the individual but the shape of creation as a whole. Maybe sin did tarnish the shine of God's handiwork and distort this perfect shape but creation as a whole has not abandoned its distinctive shape since Adam named the creatures and so discovered that the shape of all things was male and female but him. He was alone. When God created Eve, Adam exclaimed that this was at last bone of his bone and flesh of his flesh. Adam had discovered not only the shape of creation as a whole but his own lack, which God would repair forthwith, Adam having recognized that he was all alone. This is, according to those who love to tinker with Scripture, a trivial and colloquial expression of ancient history now thoroughly repudiated. We have grown past all of this, so to speak. The command to love has transcended the shape of nature (creation) and replaced it with personal desire and the right to have that desire fulfilled. The problem with this is that it also unleashes all the perversions still constrained by societies disapproval and leaves the individual to do what is right in his or her own eyes. In other words, what was once a particularly good definition of sin has become the statement that champions the new found morality of love that has no bounds.
Imagine that, St. Paul.