Bisexuality is much more prevelent and I'll tell you why.
Girls have seen gay porn and it turns them on and a lot of them will tell guys they want to see two guys together.
So a guy who was bi 20 years ago might have just fucked girls because he would figure girls would reject him if he fucked guys, but today he knows that fucking dudes will actually get him more girls so he'll be more willing to do both.