Homosexuality is a byproduct.
This is my favorite explanation, because ultimately it's about development. Why do men have nipples? Because women need them. Both men and women have the same set of genes (more or less), and follow very similar developmental pathways, and the nipple represents a developmental constraint or byproduct: mutations that knock out the male nipple might also knock out the female nipple, so the structure is retained in both sexes. Male nipples are a byproduct of a function needed by the other sex.
We might also ask, why do some men love other men? The answer: because women need to love men. (We could also propose the complement, that lesbians exist because men need to love women.) If there are pathways that can predispose an individual to find males sexually attractive, the base structure is present in both men and women, and what we have are additional mechanisms to modulate the expression of the trait in men vs. women. Just as we guys have an echo of a female attribute in our nipples, why not assume that we also bear echoes of female mate preferences in our brainsÂechoes that can't be expunged without also eliminating women's desire for men (and oh, no, we mustn't have that, I know)?
To those more versed in biology, perhaps this sort of view comes as no surprise but I have to say that I honestly had never heard a biological explanation for homosexuality that centered around the sharing of other traits between men and women before. I've read arguments before, such as Roughgarden's, that propose homosexuality as a trait which has social bonding advantages. I don't necessarily disagree with that, though I'm no biologist and what sounds reasonable to me may not to a trained scientist.
What I like about the convergence of the two ideas, that homosexuality has advantages and that it's a byproduct of shared traits across both genders, is that it neatly begins to eviscerate most arguments against gay civil rights. One of the biggest claims by opponents of gay rights, beyond any dogmatic religious taboos, is that homosexuality is not "natural". The underpinning of this mistaken assumption is that the sole purpose of sex is procreation, which is obviously not true in any biological sense. It's purely an ideological stance generally based on, again, certain religious views. Sexuality has many purposes and is both physically and emotionally gratifying, which facilitates more harmonized relations between individuals and, by extension, societies at large. That this same harmony can beachievedd between members of the same sex seems, to me, to be a perfectly reasonable hypothesis. I will say, and Myers points out, that it is difficult, from an evolutionary perspective, to completely jump on board the notion of homosexuality as a biological advantage without a proven heritability to homosexuality. Still, I think it's a compelling idea.
I have long maintained that a biological versus behavioral distinction of cause for homosexuality is moot in terms of gay civil rights. Whether homosexuality is an inborn tendency or a learned behavior is irrelevant; the government still should not be in the business of regulating the sex lives of Americans. However, since so much of the counter-argument to gay rights revolves around phony pleas to biology, I'm always happy to see science continue to make religious fundamentalism eat its own guts. The reality is that sexuality is much more complex than the gay/straight duality allows for and, unfortunately, that duality completely dominates the political discussion. A good case for the biology of homosexuality can have the added benefit of further marginalizing the anti-gay crowd by demonstrating that the differences between gay & straight really aren't so big as they appear.