I think the "lesbians" in fandom are either:
A) Actually 15-year old girls who say they are lesbians because boys don't like them enough / still have cooties.
B) Are in fandom because they are lesbians and fandom is generally more open with that.
or
C) Are "bi" because they are more open-minded because of fandom.

Also, yes, most of the ones I know tend to be "asexual" or girls who think they are really boys... e_e