This will most likely turn into a flame war knowing past threads about the subject matter, but I want to ask the community this question so here I go. *zips up flame retardant suit*
I believe that someone is capable of being whatever sexual orientation they want, whether they are a man and like fellow men, or a woman and they like fellow women. I don't think you're stuck as one side of the fence and incapable of liking the other gender, you can if you want to because you are a human being capable of free will and thought*secures flame suit further*, some may say you will not like it, but there's nothing but you stopping you from liking whichever gender you want, and being whichever orientation you choose.
What about everyone else?
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I want you to start liking the opposite gender you do now to prove your point. If you can become completely attracted to them, and not at all to whichever you are now, then I'll believe you have a valid point. However, I'm guessing my the nature of your thread that you wouldn't do that. The same way you wouldn't just change the music you like, the games you enjoy, or the people you'd want to be friends with. It's what you like and what you don't like. And while I suppose that for some people they can, over time, change what they like, it's not something that everyone can do overnight, if at all. So to answer your question with a simple yes or no, I would say that for most people, in most scenarios, no. It's who you are, and very few people are about to change who they are.