One theory about being attracted to other people of your gender, (or homosexuality) is that you are born like that. However, this is a very black and white answer to what is a very complicated matter. I believe that one can be attracted to another of the same sex without being gay or bi-sexual. For example: Presume for a moment, that you are heterosexual. You are having sexual relations with your partner. They blindfold you and after a short while they're going down on you. You are enjoying this intensely when your blindfold slips and you find someone of the same sex to be doing the job. Even if you are repulsed by the sight, you cannot deny the pleasure you were getting from it before you realised. Thus you enjoyed a homosexual encounter, even though you didn't realise. This doesn't make you gay, it can be scientifically explained with hormones and whatnot. Has anyone ever had any feelings similar to this?
Personally I believe sexual orientation isn't fixed, but fluid. That doesn't mean you can "change" it, but does it naturally change or "bend" or do things you don't expect it to? Of course it does. As far as your little hypothetical situation goes, it really is a ridiculous situation and really doesn't prove much of anything... if you are blindfolded and a member of the sex you generally aren't attracted to is doing it, but you think it's a member of the sex you are generally attracted to, it doesn't mean that you are oriented toward that person, but the person you think is doing the act. Were you to not be blindfolded and be watching the act or knowing who did the act, you probably wouldn't be attracted and wouldn't have gotten much pleasure from it. So really, I don't get exactly what about this situation is supposed to prove anything... that people can be tricked into homosexual acts? What does that have to do with that person's sexual orientation?