One only need peruse the galleries of a fetish pornography website (I don't recommend it unless you're into that sort of thing) to see how vastly sexual preference can vary, even within the broad confines of "I like women" or "I like men".
Recent posters seem to have nailed the idea that sexuality isn't a simple black and white, or even gray-scale issue.
One thing that struck me is, if you were 100% straight male (let's just pretend to assign numerical value for a moment) you surely wouldn't be able to stand watching a straight sex scene in, say, a movie, right? Despite the delight of watching a naked goddess rolling about in pleasure, you would be so totally turned off by seeing a naked male that it wouldn't work.
What if you were 95% straight and 5% gay? Maybe then you wouldn't be quite so bothered by the naked guy, and could enjoy the scene, as long as you didn't see any full frontal male nudity, or something.
By the time you get to 50/50% maybe you get as much delight from watching the guy as the girl. But then as you tip the other way, the opposite happens. By the time you're 95% gay and 5% straight you're loving the guy, but the naked girl's just palpable.
Finally, at 100% gay, you can't watch a straight sex scene, because the image of a naked woman rolling about is such a turn-off.
Given that films would only have gay sex scenes if most people were at the extreme ends of the scale, clearly most of us are somewhere closer to the middle. So basically, everyone's a little gay. (I've often thought that anyone who's really into straight porn must be quite gay, since it mostly seems to consist of intense closeups of penises!)
The same sort of sliding scale could be applied to a plethora of different characteristics, such as age. Start with a 10 year old boy and an 60 year old woman doing the horizontal dance together. Who's into that? No thanks. Two 35 year olds? I can handle that.
Then again with general attractiveness. I don't know about anyone else, but I think I'd actually prefer a sex scene with two averagely attractive people than, say, Natalie Portman and the Elephant Man.
You could make up sliding scales like this forever, using different opposing characteristics of sexual attractiveness.