The "Man Who is Thursday" gives it a shot here. It's an important question, but I think there are some flaws in the reasoning.
Thursday notes that the derogatory use of "fag" continues among male heterosexual high school students even in a place as self-consciously egalitarian as Toronto. He speculates that this contempt may have deeper roots than ideological indoctrination. If he's right, then we should figure out what it is if we want to effectively combat a form of hostility that can be violent at wrost and discourages males to adopt civilized tastes that seem too femmy.
So far, I am in complete agreement. But I have trouble with Thursday's ateiology. He says:
My own theory of male homophobia is that it is rather easily explainable in Darwinian terms. The human male is almost always pretty obsessed with having sex with human females. However, the one thing in the universe that most resembles a human female is, of course, a human male. Therefore, without some intervening factor, there is the significant possibility that many men would end up obtaining sexual release with other (very horny) men instead of trying to persuade (sometimes reluctant) women.
What Thursday is talking about is "opportunistic homosexual behaviour." It is just not the case that this is universally disdained or considered unmanly in human cultures. As Richard Posner points out in Sex and Reason, societies without companionate marriage between men and women are pretty relaxed about men who substitute male sex objects for women when women are unavailable, at least as long as those men take the active role in anal intercourse and the passive role in oral sex. A man who takes the converse roles may be considered contemptible, but not evil or threatening. A few cultures -- Periclean Athens and tradtional warrior Pushtun culture -- create a system of socially-approved pederasty, in which a younger male can be penetrated with only minimal loss of status and no loss on the older, active male's part.
As Posner also points out, Puritan-Protestant cultures which valued companionate marriage are more anxious about opportunistic homosexuality, and so created even more extreme penalties for sodomy in the case of the active partner. This is perhaps because opportunistic homosexuality is a bigger threat to companionate marriage.
At the more theoretical level, the trouble with Thursday's hypothesis is that it posits male sexual activity as being highly costly in evolutionary terms. But the premise of evolutionary psychology is that male sexual activity is extremely cheap (if not perhaps costless). Men are capable of several orders of magnitude more sexual experiences than offspring. As many animals show, as long as there is a heterosexual preference, there is no great evolutionary cost to homosexual acts.
Even if there were such a cost, it would explain only why (most) males are averse to engaging in homosexual acts themselves, not why they have a problem with other males doing so. Naively, we would expect males to want other malest to be homosexual, since it reduces intra-gender competition.
A better explanation comes from the modularity of the mind. In order to reduce infectious disease, we became programmed with a sense of disgust at unhygenic acts. The "disgust" module will be overriden by the "sexual desire" module. But if we don't feel the sexual desire, then disgust will return. This explains both the reaction of heterosexual males to depictions or even references to gay sex, and also the symmetrical view of many gay males about heterosexual acts. (Read Dan Savage.)
But I don't think this is the full explanation of why Thursday's students talk and act as they do. Why is it so important not to be thought a "fag"?
I thnk one obstacle to thinking this through right is that we tend to imagine that the only biological purpose of sex is procreation. It is obviously correct that desire for sex exists most fundamentally because a sexually-reproducing organism that did't have such a desire wouldn't be any of our ancestors. But there is no reason that sex can't play other functions as well. It can build up relationships of common action (which are obviously evolutionarily useful) and it can express status hierarchies (which also have straightforward evolutionary explanations).
In every culture, a penetrated male is reduced in status. I suggest that the median male heterosexual brain has in it a module for homosexual sex which is all about status hierarchies. Calling someone a fag (and therefore penetrated) is an assertion of dominance over them. To be confused with a "fag" is to lose status, especially where social egalitarianism prevails, eliminating the countervailing sense that cultural accomplishment grants status.
What is to be done? I'm not sure. I think we have to recognize that homophobia (I have concentrated on male hostility to male homosexuality) is part of the nature we are put on this earth to rise above. It won't wither away after the revolution, but liberal North America has obviously done a good job in socializing its boys to affirm that such discrimination is inappropriate. Of course, this has not prevented them/us from being anxious about being thought to be homosexual. That anxiety is sometimes comic, but it has unfortunate results.
Anyway, to my readers of all sexual orientations, Happy Valentine's Day!