Guys, in your obsession with gay sex, you're missing Paul's point.
Paul's Roman audience is a mix of Gentile Christians and Jewish Christians, Rome being a pretty cosmopolitan city. Chapter 1 is addressed to the Gentile Christians; Chapter 2 and the first part of Chapter 3 are addressed to the Jewish Christians. I can elaborate on that if it's not obvious.
Romans 1:18-28 makes it absolutely clear that Paul considers the big problem in the Gentile world (which is basically the whole non-Jewish world) is idolatry and not acknowledging God as God. Verses 18-23, 25, and 28 should make that obvious. The unfit minds, the darkened hearts is God's punishment for our ignoring Him and worshiping idols. That's the "why" of it.
All that other stuff that Paul mentions in Romans 1:24-31, our sexual perversions, our greed, our evil inventiveness, our envy, our slander, our murders, our strife, our insolence, our arrogance and pride, our rebelliousness, our untrustworthiness and lack of compassion and mercy, all that stuff are secondary effects of our depraved minds, not the other way around.
And knowing God doesn't just make that stuff go away, as Paul will make clear to his Jewish-Christian audience (who did know God and God's ways) in chapters 2 and 3. There is none righteous; no, not one.