I can think of a number of reasons why gay men are facinating to "study" but thats another post.
As to Nerds point Iv had a hypothesis about this and would love to hear other opinions.
It seems that to the male the height of weakness can be seen as submission to another male. In order for sexual relations to be engaged in one must submit to the other. One is giving , one is receiving OR submitting to the penetration (by what ever means). Of course if a male has a "predisposition" (ie:genetic) towards heterosexuality the thought of gay relations may seem gross for lack of a better word. But "gross" doesnt account for the (sometimes violent) FEAR of homosexuality.
Lesbians aren't scary since they're not trying to stick their penises in your mouth while you sleep at night.
Gays! The scariest of things that go bump in the night! Move over vampires and ghosts and warewolves and monsters...