An article in the Easter Sunday New York Times Magazine shows that homosexual behavior is found in hundreds of species of animals--even insects. That this merits an article, with lots of hand-wringing as to whether gay behavior is right or wrong, natural or otherwise, or indeed even worthy of explanation reveals a lot more about our culture than it does about nature. In short, it shows that our obsession with sexual preference is myopically ethnocentric.
Sexual preference is doubtlessly based on many genetic and environmental factors. Rather than being a rigid threshold, or a yes-no phenomenon, it is some sort of continuum that involves emotional preferences, social constraints or learning, as well as actual copulation and reproduction. This is so obvious it should not have to be discussed and trees don't need to be killed to print stories about it (of course, some of those trees might be 'homosexual'!).
The most important point is going to be the one least heeded because it threatens many vested, often fervid, beliefs about humans and evolution. That's that homosexuality is a problem that human evolutionary anthropology (as well as 'Darwinian medicine') have to explain. The problem is darwinian, or any other fundamentalism, masquerading as 'theory'. Darwinian fundamentalism assumes that what's here and organized must be so because of natural selection. No room for incidental traits, nor a wasted calorie.
From an evolutionary point of view it is no surprise of any kind that homosexual behavior would exist in other species, nor that it would have many manifestations. After all, even within humans it is like that. There is not a whiff of evidence that homosexual behavior is favored per se by natural selection, and clearly if the distribution of interactions and genital use shifted too much towards the homosexual, then it would be selected against (or some modified form of reproduction would replace the 'standard' forms--which, by the way, are themselves hugely variable).
The spectrum of sex and gender behavior, contrary to the oft-expressed shock that homosexuality could even exist (since all behaviors must be here because they're adaptive, and how could homosexuality be adaptive?), is no threat of any kind to properly understood evolutionary theory. If there were a single allele (a variant in a single gene) that inevitably conferred exclusive homosexuality upon its bearers, then our theory predicts it would disappear since if its bearers didn't engage in reproduction it could not be transmitted to the next generation. If complex genotypes confer an increased chance of gay behavior that reduced the bearers' number of offspring, it could proliferate only by chance, or by raising the reproductive success of closely related kin, though that's problematic because they, too, would carry the 'bad' gene. Theories of such 'kin selection' are elegantly mathematical but hard to prove.
On the other hand, sex-related behavior is manifestly complex, a mix of environmental, genetic, and chance effects. Given that, most 'gay' genotypes are unique--every case different, with very little net selection against any specific contributing allele. Just like other perplexing complex traits, they really aren't 'genetic' in the meaningful sense of the word, and fluidity in behaviors is simply tolerated by the weak selective constraints typical of nature.
But this kind of behavior may be interesting in its own right, if it could be stripped of the Just-So story kind of science that engages in excessive theorizing and hand-wringing, and we recognized our culture-bound reasons for even thinking it was particularly interesting. That we cling to fundamentalistic, or tribal, 'theory' is a failure of anthropology to educate scientists about themselves, or a reflection of a deep need for simple organizing principles to live by. And besides blinkered scientists, an understanding of anthropology should suffice to show that some people in a given culture, such as ours, may claim to be revolted by gay behavior but that other perfectly respectable cultures do not share that view. One doesn't have to like, or dislike, variations in sex or gender behavior to realize that such a personal view, to which everyone may have a right, is not mandated by any ultimate truths, and there's certainly no legitimate justification for religious bigotry in this respect.
An important bottom line is that this is not a human-specific problem in any evolutionary sense. The genetic permissiveness in regard to sexual preference is probably so complex that it has virtually no longterm evolutionary consequences or implications. If this is a sociobiological problem, it's hard to see how. Clearly, even in non-social species this has not happened, and homosexuality was present in early animal ancestors, so we've inherited that capability countless eons ago. It is not particularly important to species success, and not tightly monitored by natural selection.
Above all, there is no justification for human exceptionalism. To the extent that homosexual patterns are important to human society, it is not an evolutionary question but a cultural one. Its nature, variation, and impact might be interesting to study on their own, as phenomena that vary from culture to culture. If gay men help care for their brother's children it's an interesting aspect of social structure that is essentially irrelevant to the genetic aspects per se. The genetic mechanisms involved in behavioral differences might be of physiological interest, but they do not explain the phenomenon.
The same holds for many other aspects of social and behavioral traits to which human exceptionalism and lots of 'theory' have been applied. And it's why many people consider 'evolutionary psychology' to be a made-up kind of junk science, if it insists on tight 'darwinian' explanations for things for which the evidence such as discussed in the Times article shows simply isn't there. And it's time to stop geneticizing social behavior, even if, of course, genetic mechanisms are involved.
As anthropologists realized more than a century ago, social facts are best explained in terms of social facts (again, even if a minority of cases, or of behavioral extremes, have specific genetic explanations). Treating social behaviors as if they needed evolutionary explanations in the sense of Darwinian selection adds value judgments that are far more a reflection of preconceived notions of modern society (that is, of the scientists who are peering into behavior with Gotcha! hubris). Unfortunately, we know from clear and repeated history that such value judgments often provide the excuse for discrimination against people, with the upper classes deciding who's good and who's bad.