I don't think it's an all-or-nothing thing.
I think most people are straight, and then some are gay, and some are in the middle. It's more of a sliding scale. Some gays can be attracted to the opposite sex every now and then and some straights can be attracted to the same sex every now and then.
But I also think that it's part of you -- I don't think you decide who you're attracted to, you just decide whether you are going to act on it.
Personally, even though I am very much pro-gay rights, I have never been attracted to another guy, and never had any fantasies about it. My wife, on the other hand...
The bottom line of course is twofold: (1) Who cares? and (2) What business is it of anyone else?