Hi Melissa,
I don't remember just what this book had to say about bi-sexuality, but a very good book on the miriad variations in femininity/masculinity and sexual preferences is "Brain Sex" by Anne Moir. Its been several years since I've read it, but it goes into various studies done on maternal hormones in utero. I also, don't remember just what it had to say about genetic predispositions, but it is a very good read on the topic.
I liked Crashfrog's reply. Most of my women friends, (at least those who don't subscribe to the fundamentalist extreme aversion to homosexuality), whom I've discussed this with agree that, while they themselves might not find themselves sexually attracted to other women, it isn't aborant to them. In fact most agree that one can find many thing sexually appealing about both sexes. I think that the fact that most women can look at another woman and say "She's beautiful," or "She has a great figure" shows this also. Most men that I know find it very uncomfortable to make a general statement like this concerning other men. Many claim to be totally unable to make a judgement on whether or not another man is attractive.
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Asgara
"An unexamined life is not worth living" Socrates via Plato