One of the more common arguments against homosexuality is that it is unnatural. Clearly, the argument goes, our genetalia are designed for male-female procreation, so why sanction the supposedly deviant practice of homosexual genital stimulation? But it has always seemed obvious to me that human genitalia, and our fascination with our own, go well beyond any procreative imperative.
A tremendous amount of human energy is devoted to socialization. Our ability to cooperate in ever larger and more abstract groups has been a spectacular success evolutionarily, and socialization has been posited as the primary purpose of language. (The older, utilitarian hypothesis is that language developed as a tool for coordinating action on big game hunts, but it has been pointed out that among modern humans, something like 95 percent of conversation is social and only about 5 percent is directly utilitarian in a "pass the salt" sort of way. The more recent hypothesis is that language is useful because it allows you to "groom" more than one person at a time.)
Our sexual drives are also intensely social, far more oriented towards pairing up with appealing partners and satisfying our own desires than towards making babies. Is there any biological reason why sexual socialization shouldn't follow the same patterns as conversational socialization in terms of gender distribution? Whatever the specific details might be, it seems clear that homosexual erotic desire is common enough in humans that to label it "unnatural" is to distort the very concept of what we mean by nature.
What brings all this up is a BBC report on a natural history exhibition in Norway entitled "Against Nature? An Exhibition on Animal Homosexuality," which documents homosexuality and even long-term homosexual pairing among species as diverse as penguins and bonobos. The latter, in fact, are probably the closest relatives we have in the animal kingdom, and they appear to be wholly bisexual.
Of course, I've often thought that the whole debate over whether homosexuality is natural or not is beside the point. Lots of urges are clearly natural — desires to smash the heads of people who anger us, desires to have sex with attractive strangers, desires to defacate when we're in inappropriate settings — yet we proscribe them. Much of the point of civilization, in fact, is to teach people how to repress their natural urges. (On this topic, Freud was absolutely correct.)
So the question of whether homosexuality should be socially acceptable shouldn't hinge on whether it's "natural." A gay gene wouldn't give homosexuality any social legitimacy, any more than a genetic predisposition to pedophilia would make such acts acceptable. What should give homosexuality social legitimacy is our liberal tradition of support for individual freedom. Homosexual acts are consensual and do not impinge on anyone's liberties. As such, there is simply no legitimate reason to ban such behavior. Indeed, genuine support for liberty requires that we protect the rights of all adults to engage in consensual homosexuality as they see fit, just as it requires that we protect other rights.
(This doesn't resolve the question of whether gay marriage should be legalized. I personally believe that the state should get out of the marriage business entirely — that's really for individuals and their religious institutions — and simply allow domestic unions among any two persons, including close relatives, with shared property, shared child custody and all the other attendant rights. I recognize, however, that this is totally unrealistic. As such, I believe that gay people should have the same rights as straight people to choose their legally recognized life partners, and we should call it by the same name when they do.)