I tend to think of sexuality as a gradient of preference for one sex or the other.
With M for male and F for female, you could illustrate it like :
M--------------------------------------------------F
And then, maybe NumberNineDream would be here :
M--------------------NN------------------------------F
And adidasss might be here :
M------A--------------------------------------------F
We like to describe and classify ourselves often as absolutes like "***" or "straight" and most of us adopt those ideas - live up to them and make them part of us. However, in reality I think it's a gross simplification of the truth.
Just look at our closest animal relatives, the bonobos. Anything goes for them regarding sex
That doesn't mean the same should necessarily go for us, but of course the way we express our sexuality is severly constrained by culture - not just how we practice it but with who. Without cultural constraints, maybe homosexual behaviour would be common even for people who have a clear preference for the opposite sex. It is with most other social mammals out there.