I recently read an article in Psychology Today regarding the differences between men and women when it comes to bisexuality. One part of the study interviewed women and who defined themselves as gay or straight. Not only were straight women much more likely to have had sexual experiences with the same sex than straight men, but gay women were also much more likely to have had sexual experiences with the opposite sex than gay men.
The second part of the study involved measuring the sexual arousal of straight men and women when shown homosexual porn, and vice versa with gay men and women. (This was done scientifically, not subjectively, by measuring blood flow to certain regions.) Again, straight women were significantly more likely to be aroused by the same sex, as well as gay women by the opposite sex.
The study seemed to indicate that in general, female sexuality tends to be more fluid, while male sexuality more rigid. Often people think this might be because of men's social conditioning, i.e. is less socially acceptable for men to have same sex experiences. However, this would not account for the fact that gay women were much more likely to have opposite sex experiences than gay men.