> Classical Greece springs to mind. Possibly high Roman culture also.

Athenian culture encouraged sexual relationships between grown men and teenage boys - but mostly looked down on relationships between grown men. And, in Athens, same-sex relations involving women were less acceptable than those involving men. (Sparta was quite different in that regard.) For most ancient Greeks-and most ancient Romans too-sex was moral when it reflected societal power relationships (man-woman, man-boy, man-slave), immoral when it inverted them or ignored them. Consent was largely ignored. Many Roman men were repulsed by female homosexuality since they saw it as a woman attempting to usurp the dominant male role. (There are possibly some exceptions, such as Catullus, but those exceptions don’t make the generalisation untrue.)

Ancient Greek and Roman culture was a long way off contemporary mainstream contemporary sexual mores. It was very different from traditional Christian sexual morality, but different in a completely different direction from contemporary Western society.

As much as I agree with your point that many other cultures differ from mainstream contemporary US sexual mores,

the cautionary note on all these stories of ancient morals is the obligatory note:

according to ...

WRT Rome, for example, we are well versed in the opinions of Marcus Terentius Varro, of Petrarch, Cicero, Ovid, Virgil, et al.

We know first hand what a good section of relatively elite citizens with the time and education to write extensively thought, those that set aside documents well enough to stand the passage of time given good fortune.

Athens less so.

In both cases, though, it bears mention we have the expressed opinions of select fortunate elite on the morals of others.

Catullus, as an outlier to some degree, is a good example of the breadth of opinion that we're largely unaware of, we can imagine other writers less outrageous that we have no record of, and other writers again that were perhaps even more lewd and outspoken.

I'm not here in defence of Caligula but it's worth mentioning that he may not have been quite the sexually perverse total asshole that Cassius Dio and Josephus (the primary sources) made him out to be.

Neither Greece nor Rome is a valid example. There were not the restrictions we're used to. But taboos abounded, particularly e.g. the stigma against cunnilingus. The Romans in particular gave us the phrase Quis custodiet ipsos custodes which was motivated by some noblemen's jealous guardianship of their wives.