As someone interested in genetics and social behavior, here was a passages that caught my attention in the Posner decision:
Although it seems paradoxical to suggest that homosexuality could have a genetic origin, given that homosexual sex is non-procreative, homosexuality may, like menopause, by reducing procreation by some members of society free them to provide child-caring assistance to their procreative relatives, thus increasing the survival and hence procreative prospects of these relatives. This is called the “kin selection hypothesis” or the “helper in the nest” theory.
The passage includes a citation to this “responsible popular treatment” of the topic. That article is better, in that it articulates a number of different explanations for how homosexuality being “inborn” (as Posner puts it) is not incompatible with evolutionary theory. What’s interesting is that Posner’s decision singles out for attention the weakest argument of the lot.
What’s weak about it?
Because homosexuality is not at all like menopause in a way that is extremely important for talking about evolution. Namely: every woman who lives long enough goes through menopause, at least in the key sense of eventually becoming infertile. It’s a feature of the species. In contrast, only a small percentage of people are gay. It’s unusual, not universal.
You can come up with some kind of equilibrium argument for why the benefits of gay uncles–note: the theory is just about gay men–imply a low-but-nonzero population frequency. (It’s a bit like how people have offered adaptationist explanations for left-handedness.) For homosexuality, the math of that is sufficiently daunting that the authors of the cited article don’t even try, but instead fall back to the conclusion that this may have enhanced fitness in combination with other things. In other words even if you do believe in this argument, you need to pick a second explanation as well to make it work.
In any event, for other reasons including those mentioned in the article, being gay does not have to have served some higher adaptationist purpose in order to be “inborn.” But also, and ultimately more importantly, the right of gay people to marry shouldn’t be in any way be contingent on sexual orientation being understood to be either “inborn” or “immutable.”