butch women challenge male privilege in a way that femme guys don't. i think it might be easier for a woman to accept a guy who can be almost like a girl friend, than a woman who is more like a guy (who'd ever want to be a man??). feminists insist on women being at least equal to men, and a butch woman would go against some of their beliefs. men either don't find butch women attractive, or don't dare admit that they do.
there is an extreme pressure on women to be feminine, and it comes from both binary genders. i don't see the same pressure on guys, other than when his girlfriend complains he isn't manly enough. when a man shaves his legs, it might be weird or queer, but when a woman doesn't, it's suddenly considered ugly and unattractive.
i can find pretty many reasons why women don't want to present more masculine, which don't have to do with their own preferences.
i especially don't like how lesbians in the media are always femme. most probably are, but why not try to find those butch ones? probably because both men and women don't like the idea of a woman not being well groomed and proud of her own femininity.