Have you ever heard of Occam's razor?
More seriously, there are dozens of theories attempting to explain homophobia (or, more accurately, heterosexism), most of which make more sense than the above but none of which will ever be definitive, because different people hate for different reasons and because some pervasive human beliefs are so old that they have been lost in prehistory. Basically, though, the answer's going to be a mixture of societal discomfort with sexuality in general (heterosexual intercourse excepted, what with the carrying on of the species thing), sexism, and the need to keep categories neat and distinctions distinct. I don't think it's hard to understand where homophobia might come from. It's why we can't make it go back there that's bothering me.
Andrea is home with the kids and going stir-crazy. Write her a letter! Ask her a question! Send her your tedious e-mail forwards! On second thought, don't do that. Just ask her a question.