A commenter writes:

[H]omosexuality is not natural in that it clearly functions against the survival of the species.

I've heard versions of this article before, so I thought I'd pass along a few reactions.

1. To begin with, I take it that the commenter isn't asserting that tolerating homosexuality (or recognizing same-sex marriage) actually jeopardizes the survival of the species. The overwhelming majority of people seem hardwired to be heterosexual, at least when opposite-sex partners are available. There's some malleability of sexual practices -- bisexuals, for instance, might be movable towards focusing more on heterosexual relationships or towards focusing more on homosexual relationships. But there seems little reason to think that even total equal treatment of homosexuals and heterosexuals would actually cause the species to die out.

2. The claim must therefore be that homosexuality is not natural in that (a) it diminishes the birth rate, or (b) if everyone were homosexual, the species would die out (the theory being, I take it, that artificial insemination would be cumbersome and rare enough that it wouldn't compensate for the problem).

Yet this is very far from any normal definition of "natural." Infertility is natural, even though it satisfies both (a) and (b). People's desire for some time free of the burdens of childrearing is, as best I can tell, quite natural (as is the desire for pleasure more broadly). Yet it too satisfies both (a) and (b); in fact, I suspect that this is responsible for declining birth rates far more than is the toleration of homosexuality.

3. More broadly, I agree that the natural world has created, through the process of natural selection, organisms that tend to be successful at reproducing themselves. But this doesn't mean that any behavior traits that reduce reproduction (or that, if universal, would eliminate reproduction) are unnatural.

4. Finally, to my knowledge it's not clear that the incidence of some amount of homosexuality diminishes the overall societal birth rate -- in fact, if strongly homosexual orientation is genetically linked, that's reason to think that those genes carry some reproductive advantage to gene carriers, or at least have little reproductive cost. I'm told that there's a hot scientific debate about this, and I'd be delighted if those who know something about the debate can speak to this in the comments. But my point is simply that observable conditions (whether sickle-cell anemia, menopause, or homosexuality) that seem to decrease people's aggregate reproductive fitness may through indirect channels actually increase aggregate reproductive fitness; and we should be careful about just assuming otherwise, especially when the condition seems to be genetically linked.