I don't really think it's mainly due to social convention: even among people who are otherwise willing to go against the grain of society, there is still a lower incidence of bisexuality among men from what I've seen.
There was an interesting study done on male bisexual response a few years back, which
this article describes along with some countering criticism. I don't buy into the idea that there's no such thing a a male bisexual any more than I buy into the idea that we're all a little bit bisexual. I do think, broadly speaking, that sexuality is a bit more innately social for females which does lead to a more fluid attraction. Of course, this is mostly speculation on my part, so I reserve the right to be proven wrong
