Romans 1 is a text that frequently surfaces with respect to the question of sexual ethics, especially same-sex sexual activity. Obviously there is much ink that could be (and has been) spilled on this topic, but here are a few brief observations on the text of Romans 1-2.
1. There is a connection between idolatry and sexual immorality
2. Sexual immorality is not the cause of God's judgment, it is the result of God's judgment.
3. There is a logic of "exchange of otherness for sameness" in the text.
People exchange natural sexual relations for unnatural ones. Men stopped properly engaging with women (the other) for other men (the same).
This makes sense of why Paul would mention same-sex sexual activity here--not because it's the sin of all sins but because it maps well rhetorically onto what he's saying. Of course, the fact that it maps well on to his rhetorical point doesn't mean it's not sin.
4. The terms "natural" and "unnatural" can't be equated with 21st century social-cultural constructs, such as "sexual orientation."
Now, it seems to me that this line of reasoning would make sense if one could argue that Paul (or any first-century Jew for that matter) means by "natural" what contemporary thinkers mean when they talk about "orientation." But having read a number of scholars on both sides of this discussion, I haven't seen a strong case for that point. It would be analogous to arguing that the author of Genesis was thinking about modern scientific issues when they wrote Genesis 1. We're talking about thought worlds a couple millenia away. It seems much more probable that Paul is making a basic point that draws on Genesis 1-2, Lev. 18, and the words of Jesus regarding normative human sexual activity. The natural order, as set out in those texts, points to monogamous male-female marriage as the norm.