Plus, I think it has a lot to do with there being so many bisexual and bicurious women. Most of the "lesbians" I know were heterosexual women who were introduced to the lesbian thing and decided that women were less a hassle than men. It doesn't make them any less bi-sexual, it's just that they've vowed off off men. Since I've met enough of these people to make them statistically significant, I think that men look at it and say "see, she likes guys even though she says she doesn't, why don't you?"
And even more controversial is since I've seen enough people "change" sexual orientations that people think with enough convincing you might try it and like it. Heck, my ex-gf tried women, then didn't like guys anymore....so it works both ways. Sadly.