I’ve had the same question before, I’ve wondered about it enough to create a few possible answers and some more questions. I can see two distinct parts of homophobia (there may be more, but I can’t think of any off hand).
Part A: Homosexuality is unnatural. Sex is about reproduction; gay sex cannot result in reproduction.
As far as I’m concerned, this is about 75% hogwash. Sex is not all about reproduction; contraception is a very old and accepted technology in those cultures that aren’t laboring under this misconception. As a non-Christian, it’s hard for me to believe that entire religions full of people are unnatural and/or going to hell. Furthermore, gay people do reproduce, parent, and (want to) get married.
The other 25% of “gay is unnatural” is sort of true, since just about all human sexuality (other than simple copulation) is unnatural. Did boxes of chocolates, senior proms, and Divorce Court evolve alongside Australopithecus africanus? I suppose this question doesn’t impress the creationists, but maybe it will help some people along.
Another part of the 25% is the question, if our (hetero-) sex drive is such an important part of the propagation of our genes, how come homosexuality hasn’t programmed itself out? Possible answers (don’t know if I believe them, but I thought them up): genes are nearly irrelevant to homosexuality; sex drive (homo- or hetero-) is more important to mom-and-dad bonding than it is to actual procreation (folks would still do it even if it weren’t quite so much fun, but they might not be as choosy about their partners?)
Part B: Homosexuality is a failure of morality and/or a triumph of hedonism.
This is about 100% hogwash (margin of error: 3 percentage points). In fact, this notion offends me as a heterosexual. Sure, I’d like to rob a bank and get rich, but my morality prevents me from doing so. Sure, I’ve been made at kids and dogs and wanted to slap them, but my morality prevents me from doing so. No amount of immorality is going to make me think Tom Cruise is better looking than Nicole Kidman (or Rosie O’Donnel for that mattter). This is because homosexuality is an orientation equal to heterosexuality, not a deep-down hidden desire underlying a superficial straightness.
Here’s a thought: To consider homosexuality a moral failing is actually a Freudian slip. People who feel this way are closet-gays who recognize on some unconscious level that their heterosexuality is a paper-thin veneer, nowhere near as strong as their latent desire to smooch Tom Cruise (I assume these are mostly guys; could be wrong).