This is fascinating stuff! I've been thinking about this and I think part of the reason that fandom appears straighter than it may actually be is that, at any given time, some percentage of the women in fandom who are queer may be living relatively heteronormative lives (at least to outside observers) - even if these women are vocal about their sexuality, there are people, both inside and outside fandom, who will insist that these women are "pretending" to be queer (essentially denying these women the right to define their own sexualities). In other cases, women may be reluctant to share their sexuality with someone they don't know, and depending on how rigorously the analyst collects her/his data, the analyst may just assume that someone is straight because of her/his current choice of partner.

This may be true of men in fandom as well, but the examples I can think of are bisexual women who are in monogamous relationships with men. . .

(And of course this happens in the offline world as well - bisexuality tends to be erased, because someone's "real" sexuality is assumed to be defined by whoever her/his current partner is. If you're in a relationship with someone of the opposite sex, you're straight (and any prior same-sex experience is assumed to be "experimenting") and if you're in a relationship with someone of the same sex, you're gay (and any prior opposite-sex experience is assumed to be reluctance to come out of the closet or confusion.)