I suspect that the real reason is far more complicated than anyone is willing to deal with.
I suspect that nearly all guys, straight ones included, have had at least one fantasy involving another guy. Face it: guys love their own dicks with a passion that's both endearing (it makes them simple creatures) and frightening (Steven Tyler comes to mind). It's a natural impulse to share something you love with a friend or loved one. If that other person happens to be of the same sex, though, you're taught that it's wrong. Thus you begin to hate yourself for having something that gives you so much pleasure. It gets messy after that.
I'd also like to know why most sports involve sticks and/or balls, and why most sports are still gender segregated (not to mention locker rooms). I think that node is linked to this one.
Yo, blowdart, you'll still have 4 breasts with two gay men ... just not as big and pillowy as you want. Don't force your big-tits philosophy on me!! :)