> And anyway, it’s also quite common for straight men to have had gay experiences. ... A equally big puzzle is all the non-procreative sex that straight people have. That one doesn't get much research attention though, because straight people aren't considered to be an aberrant case.

I mentioned this in my previous comment: "Most sex among humans (and hominins in general) doesn't produce offspring, ... That's fine. Sex isn't a limited commodity. Sex is abundant."

I think this aspect of human sexuality does get a lot of attention in research; I mean, it's absolutely central to human reproductive behavior. Concealed ovulation, menstruation, extraordinarily high male parental investment for a mammal, extended female sexuality, female copulatory vocalization combined with monogamy, monogamy combined with substantial sexual dimorphism: human sexuality is weird, and every aspect of that sexual weirdness is profoundly connected to non-procreative sex. I mean, we're weird in lots of ways, but sexually speaking, we're total freaks. We're so strange we're practically birds. Except for the sexual dimorphism thing. Even mallards aren't that weird.

And it's wonderful! But very puzzling.

> It fails to recognize that procreation is just one function of sex. It’s like being surprised that people use their mouths for kissing as well as for eating and talking.

The puzzle is not that some organisms engage in non-procreative sex, though; that's a lot more common than bizarre phenomena like mammalian monogamy or concealed ovulation. It's that some organisms, especially males, voluntarily refrain from procreation. There aren't a lot of organisms where that happens: sheep are one example. From an evolutionary point of view, it's a real challenge to explain how instincts for such behavior could be conserved.

The whole naturalistic fallacy you allude to, along with all the Catholic peccatam contra naturam nonsense, definitely contributes to the confusion.

> male homosexuality - not because it’s any more in need of an explanation than any other kind of sexual behavior

It is, though. Female bedbugs, like female organisms of many species, can reproduce without ever voluntarily copulating. (Bedbug penises are the stuff of nightmares.) Even in a species where copulation is 100% voluntary, female homosexuality has to reach a pretty high level before it starts to diminish ω, especially in a high-parental-investment species; you could imagine an elephant species where cows frequently mount other cows and perform oral sex on them* every day, but only have sex with bulls three times per estrus, and only go into estrus every five years. So in every five-year interbirth interval, cows of our hypothetical elephant species would have sex with other cows some 1500 times and bulls three times. The females of this hypothetical species would be 99.8% homosexual. But they'd have just as many offspring as cow elephants of actually existing elephant species.

More to the point, in this hypothetical elephant species, a less-lesbian mutation that caused a cow to copulate with bulls ten times per estrus instead of three times would increase her number of offspring only very marginally, if at all: in the cases where the first three copulations failed to produce offspring, she'd save herself an estrus.

By contrast, a 99.8% homosexual bull elephant would have... a lot less offspring than a regular bull elephant, as long as there's an occasional Casanova bull running around engaging in extra-pair copulation. Not 99.8% less offspring, because there aren't that many cow elephants around (there's no possibility of a Genghis Khan elephant), and elephants practice mate guarding. But a lot less. Maybe 50% less. And being even a little more heterosexual would increase his ω a lot.

So, aside from the knuckle-dragging Victorian factor, there are also totally legitimate ev-bio reasons that exclusive male homosexuality is surprising and challenges naive ev-psych theories more than exclusive female homosexuality.

> None of the theories that have been proposed is really very plausible.

Agreed.

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* Lesbian elephant trunk sex is totally a thing in real life, just not as prevalent as in this scenario.