I've found that one reliable way for a person to determine his or her true sexual preference is via the sense of smell. It was always clear to me that I have no interest in males, but the clincher has always been that they just smell wrong. Pheromone incompatibility, maybe.

So if you're having doubts about your sexuality, try this test: Select a member of the sex you're unsure about. Make sure that he or she is at least inoffensive to your other sense. Get close enough to have a good whiff, preferably while the other is in a sexually excited state. If you don't mind the smell, and indeed want to get even closer, that's a pretty good indicator that you're sexually attracted to members of that sex.

I've had a number of people test out this theory -- straight, gay, lesbian and bisexual -- and they've all told me it works for them. Let me know if it works for you; give me more data points!