Because most of the time when people think 'lesbians' they think this:
It's not so much about being open and accepting of homosexuality as it is indulging in an immature male fantasy. For one thing it's worth remembering not all women look like Natalie Portman or Mila Kunis, and certainly not all lesbians. There are fat lesbians, scrawny lesbians, lesbians with wierd teeth, lesbians with greasy hair... the point is, not all women look like supermodels.
It's also a wierd fantasy anyway, if they are lesbians and are having sex with each other, they probably aren't at all interested in YOU.
But no one ever thinks about those women when they think about lesbians, and in modern popular culture you rarely see depictions of lesbian couples or women engaging in sexual acts upon each other WITHOUT them also looking like sexual dieties. If Black Swan had starred say, Betty White as Nina and Queen Latifa as Lily, people would probably not find the particular scene as attractive.
Thing is, it's not so much about women in general being accepted if they want to sleep with other women. Rather it's the image we hold up that's 'acceptable', because the idea of two attractive women sleeping together is something that we like because of the fantasy it provides us, not the freedom and love that it would provide others. It's the reason why we almost seem to expect girls to have no problem making out with each other, or having a naked member of their gender start rubbing against them. It's gotten to the point now where we just assume that they (meaning even if they aren't actually lesbians) should be comfortable with it, even though it's entirely reasonable for it to be the equivilant of a man being expected to be comfortable making out with another man or by extension having a naked man rub against him.
There's this one dialogue scene in the movie 'Chasing Amy' that pretty much summarizes the whole thing:
As an attractive lesbian, the girl Allissa has no need to fear any prejudice because society deems it 'sexy', provided that she lives up to an immature sex fantasy there will be no anger against her. The man talking meanwhile is a 'reviled gay man' because men do not find him attractive and because it's not universally accepted as sexy by a largely male dominated media, he still gets persecuted.
Which is actually why I often wonder:
If there are SO many depictions of women doing other women in men's television, film, games etc... why can't there be depictions of men making out with other men in woman's media? I mean surely there have to be a few Twilight fans who wanted to see Edward and Jacob make out on screen.
Given that they are both somewhat attractive men, one could argue the same logic applied in Black Swan with Miss Portman and Miss Kunis would also apply here. If women can have sex on camera for the sake of a male audience, why not the other way around?
I guess my final thought here is that if we could make male on male relationships in media as common place and 'sexy' as woman on woman relationships, maybe there wouldn't be as much of an issue anymore. After all I know straight women with husbands who still think Natalie Portman is gorgeous and thought that one scene in Black Swan was sexy as hell.
Maybe one day we'll hear from men with wives about how sexy that scene where Peter and David got down was.
My argument is that if that happened, there would truly be a fair balanced society. With no gender double standards and no sexual orientational discrimination.
