And traditional behavior gives us an imperfect window into the economics of the past, which is what’s under discussion when we talk about historical selective fitness.
I think we should keep in mind just how far back we’re talking. I’m not saying we inherited homosexuality from our common ancestor with the modern fruit fly, but at least our common ancestor with other great apes. Framing the question as why would it be selected for in the context of human societies is probably wrong, when what we want to know is why it wasn’t sufficiently selected against given it already existed (I doubt we’ll ever figure what advantage it gave the proto-ape whose social structures we’ll never know). Once a trait already manifests in 3% of the population, it takes work to get rid of it, and even within that 3%, it was doubtful the case that 0% of them reproduced while 100% of heterosexual men reproduced. I’m sure it wasn’t exactly parity, but it’s possible there is no explanation in terms of the organization of human societies except for we’re really optimized to enjoy sex, sometimes that wire gets flipped, and it doesn’t provide an advantage, but it also doesn’t give enough of a disadvantage to completely disappear within 300,000 years.
Don’t forget also, that if some gene combo is necessary but not sufficient, and requires other developmental factors to manifest that don’t manifest in your brothers and cousins (which seems to be the case if it’s only 20% between twins), then when they reproduce, even if you don’t, the gene still gets passed on. Take me, for example. I’m not gay, but I am sterile and don’t want kids anyway. Nonetheless, I have 3 sisters and 13 cousins that have had kids so far. Without doing the exact math, off the top of my head I’m guessing at least 80-90% of whatever I’m carrying made it to the next generation.
Edit: Also, one last thing is we don’t know the prevalence in the ancestral population. Given it’s roughly 100% bisexual in such a closely related other species, it could have been fairly high in the common ancestor, obviously not 100% obligate, but more than 3%, and it actually has been selected against, a lot, just not enough to get us to zero yet.
I think we should keep in mind just how far back we’re talking. I’m not saying we inherited homosexuality from our common ancestor with the modern fruit fly, but at least our common ancestor with other great apes. Framing the question as why would it be selected for in the context of human societies is probably wrong, when what we want to know is why it wasn’t sufficiently selected against given it already existed (I doubt we’ll ever figure what advantage it gave the proto-ape whose social structures we’ll never know). Once a trait already manifests in 3% of the population, it takes work to get rid of it, and even within that 3%, it was doubtful the case that 0% of them reproduced while 100% of heterosexual men reproduced. I’m sure it wasn’t exactly parity, but it’s possible there is no explanation in terms of the organization of human societies except for we’re really optimized to enjoy sex, sometimes that wire gets flipped, and it doesn’t provide an advantage, but it also doesn’t give enough of a disadvantage to completely disappear within 300,000 years.
Don’t forget also, that if some gene combo is necessary but not sufficient, and requires other developmental factors to manifest that don’t manifest in your brothers and cousins (which seems to be the case if it’s only 20% between twins), then when they reproduce, even if you don’t, the gene still gets passed on. Take me, for example. I’m not gay, but I am sterile and don’t want kids anyway. Nonetheless, I have 3 sisters and 13 cousins that have had kids so far. Without doing the exact math, off the top of my head I’m guessing at least 80-90% of whatever I’m carrying made it to the next generation.
Edit: Also, one last thing is we don’t know the prevalence in the ancestral population. Given it’s roughly 100% bisexual in such a closely related other species, it could have been fairly high in the common ancestor, obviously not 100% obligate, but more than 3%, and it actually has been selected against, a lot, just not enough to get us to zero yet.