Because mainstream masculinity breeds insecurity about one's sexuality.
I grew up through middle school with that, and I'm out to change that in any possible way in my lifetime.
As a man, I resent how our culture is so much more restricted than that of women. When I say this, I'm not saying men need to open up there sexuality (such as the popularized young bi-curious/sexual woman): only that we need to actually be secure about our sexuality. Acknowledging the attractiveness of another male doesn't make you gay: your sexuality determines that. And for some reason I've yet to pinpoint, being a gay man is stigmatized. Could be the aspect of gay sex in which (at least) one of the men is in a more "female" role and femininity is still viewed by mainstream masculinity as weak...
I blame it on men's lack of honest and critical communication with each other, hell, its something I still need to work on since I still find it uncomfortable for some reason... but change isn't easy, right?