I have a hypothesis on why hetero men have such a visceral reaction to homosexual acts. One, I agree with cwilbur that a portion of it is macho posturing due to insecurity and fear. The thought of being gay can be really scary to some.
Also, in broad evolutionary terms, the male objective has been to induce pregnancy as frequently as possible in order to ensure reproductive success. Therefore, where men are concerned, homosexual activity is reproductively “wasteful”, i.e. time spent away from coitus, time that the male is not siring children.
Women, on the other hand, tend to be more sexually open. The contrast of male and female reactions to homosexuality interests me because, conversely, females have large gaps where coitus is reproductively “wasteful” due to pregnancy, not to mention most of the time when they are not ovulating. I would say that this gives women a different sexual M.O.; they have a great deal of “free” time where reproduction is not the main focus of sexual activity.
It’s interesting because males seem more focused on the orgasm, while women tend to function on a more emotional level. Women focus more on personalities, relationships, feelings; it just makes sense to me that they would be less stuck on gender, more understanding of the emotions involved in relationships. Also, it is not uncommon for women to be unable to orgasm through coitus, or they may be with a partner that doesn’t give equal priority to her pleasure.
With all that in mind, it seems natural to me that women might be open to non-traditional sexualities. It’s pretty easy for most men to orgasm, but it’s a bit more complicated for women. It takes some learning about what works and how to go about it. Exploration and sexual plasticity may have actually benefited women in this sense.
I realize I’ve generalized here, but hopefully you get the idea. It’s late, I babble.