One sublety that is never discussed is who the laws of the Old Testament applied to.
Rabbinic Judaism even before the time of Christ was of the opinion that the "Law" of the Torah applied to the nation of Israel alone.
For example, in Deuteronomy, there is a discussion of the fact that the law against worshipping idols pertained only to the nation of Israel, and that, in fact, God allowed the other nations to worship idols. God did not hold them accountable for worshipping idols.
You could make a technical case for the rule about homosexuality only applying to the nation of Israel, under this theory. So, under this theory, there was no universal law against homosexuality.
Rabbinic Judaism fixed that loophole, but I don't know when. The rabbis had to account for God's judgment against the world in Noah's time. They figured there must have been a covenant -- not recorded in scripture -- that was violated to warrant God taking that action against mankind. They apparently tried to reverse-engineer what that covenant was, and came up with, among only a few other rules, a universal prohibition of homosexuality. I seem to recall that they figured that prohibitions of idol worship, adultery, and murder were also on the short-list of God's commands.
This is not an irrelevant topic for New Testament study. The question is, on what basis did Paul not require circumcision of new Christians? or, what was the basis for him not requiring observance of the law (instruction) of the Torah by new Christians?
Was it just some direct inspiration of the Holy Spirit? or, was Paul applying some rabbinic formalism that he might have been acquainted with, as a Pharisee? or, was it perhaps a combination of influences?