> All of this stems from religion, yet our system is supposed to have a separation of church and state.
That phrase appears nowhere in the U.S. constitution. The first amendment guarantees the free exercise of religion (which includes voters having religious motivations for supporting otherwise permissible laws). It also precludes regulating state established churches, which were a thing at the time. (The more expansive modern interpretation favored by some was created out of thin air in the 1940s by a former KKK member: https://scholarship.law.upenn.edu/cgi/viewcontent.cgi?params... (pp. 588-589) It’s one of the shoddier Supreme Court opinions, basing its understanding of the First Amendment on the comments of Thomas Jefferson, who was in France when the amendment was drafted. Jefferson was always a Francophile and favored the revolutionary French view of religion—which does embrace a separation of church and state—but that has little to do with what the constitution says.)
All that being said, even the more expensive view of the Establishment Clause doesn’t prohibit legislators from having religious motivations for laws otherwise within their power to enact. In that context, religion is no different than any other ideology (socialism, capitalism, etc) that might motivate legislative behavior. “Sex is shameful” isn’t any more of an improper basis for legislation than “free markets are good” or “society a an obligation to support poor people.”