As Kinsey described, sexuality is on a 6 point scale. Exclusively heterosexual at one end, exclusively homosexual at the other. But 4 in-between. In this sense only then, bisexuality is more normal than either extreme.
If we go by how many admit to sexual behaviours this will obviously change since most will identify themselves as heterosexual. But if we also ask if they've ever had a homosexual experience, fantasy, looked at and enjoyed homosexual porn, the answer changes significantly. So how many homosexual experiences before identifying as heterosexual doesn't make much sense?
According to this non-scientific poll's results (but with over 50,000 participants,) only about 33% are exclusively heterosexual, 10% exclusively homosexual, and 57% some sort of bisexual.