I think it's because of societies male-centric viewpoint foremost.
female bio-sexuality and lesbianism is seen as kinkly or experimentation, and usually is not even thought of as gay because it's being viewed from a heterosexual male perspective.
on the flip side, believe it or not, the same POV is dictated towards men. Sexual portrayal of men in media is generally far more appealing to homosexual males than it is to straight females.
Also in our society men tend to be attracted to the concept of sexuality, while women's sexual attraction is situational.
For instance, for most women viewing pornography there preference is female/male, female/female, male/male, female solo, male solo.
For men it's female/female, male/female, female solo, male/male, male solo.
Women are attracated to the act of sexual intercourse and stimulation, more than sexual exhibition.
For both sexes an attractive, sexy dressed person is generally more desirable than a less attractive nude or less clothed person. Although it skews higher in personal preference for individual woman than for an equal number of men if they were polled on the subject.