For me it's very simple. One of my best friends, who also happens to be my cousin, is gay. We grew up together, and during puberty, when we were both trying to figure it all out, we both experimented. I guess each of us was trying to convince the other that "my way is best", but what it really did was give each of us a clear read on what our orientation was.
The beauty is, both of us can appreciate what the other finds attractive, or sexy, or hot, even though neither of us is inclined to change. So for me, it's 99% straight, 1% gay - and for him it's 99% gay, 1% straight. (Yes, we've discussed it.)