I disagree with people saying that gay guys do it because there are les connotations attatched to being 'bi' personally, untill proven otherwise, I hold bisexuals to be pretentious and greedy - not on all that much foundation, but I can't help it.
I myself have called myself bi on a few occasions, but to me, it was simply a matter of liking to be open to anything life throws at me: My own, slightly less pretentious way of calling myself pansexual (or whatever the one I'm thinking of is called).
It was being converted in my head from a simple question of "What gender person do you actually like to have sex with" into a question of how open to anything new/frowned upon I was, thus (despite only ever being with 2 girls in my life, 1 of which I was not even remotely attracted to - In comparison to many, many guys) I claimed myself to be bisexual.