All I've come up with is a half-formed joke about how human females really are intrinsically attractive after all.

It is interesting. IME in real life and in OkCupid, female self-identification as bisexual correlates quite strongly with the geek/liberal/poly/kinky meme complex (edit: mirroring your experiences, didn't read carefully enough). Out of my top matches in OkCupid, over 80% of women interested in men seem to self-report as bisexual.

However, also IME, bisexual identification usually doesn't imply being biromantic! Many of those women have had, or would like to have, sexual experiences with other women, but still may prefer men in romantic relationships almost exclusively.

FWIW, I support adding a question about romantic orientation in the next survey.

I initially misparsed this as "the female bisexuality rate is as expected." I see that isn't what you meant, but had to re-read two or three times. Just FYI.

Grammar modified to be clearer, thanks for pointing that out.

It seems that women are borderline bisexual by nature. For example, heterosexual women are significantly more likely to want to dance with other women than heterosexual men are to dance with other men, and the same thing is true for all sorts of other activities that have some kind of borderline relationship with sexual activities. So perhaps there is a kind of implicit bisexuality there which is more often made explicit in the case of Less Wrong women than other women, perhaps on account of higher introspection or the like.