The best analogy I have come up with is this:
When my brother and I were really young, we were both ambidextrous (means you can write the same quality with both left and right hands). We didn't want to be the same, so, we came (or at least he did, as he's the oldest) to a conclusion; he wanted to be left handed and therefore I would be right handed. And this was fair enough, it didn't make a huge difference, and could have gone fine either way.
Now I'm grown up and whatever, if you ask me to try and write something with my left hand, I find it uncomfortable, awkward, and I'm really bad at it. I'd much rather use my right hand. Yes, I could force it if I absolutely had to, but I really don't enjoy it, and would much prefer using my right.
This is as close as I can come to explaining sexuality. Once something happens for it to be decided, then that's what 'sticks'.
You can, in some ways 'choose' but in reality you do what is most comfortable, and there becomes a point where one is uncomfortable or one is significantly more comfortable. For some people, they can still write fine with both hands, some can't even write at all.
(bisexuality, asexuality)
You can experiment all you like, even force it one way or another, but you'll find that most people have preferences, and that's fine.
The majority of people are right handed, but that doesn't mean being left handed is wrong or worse.
*sigh* I am not even sure if my words are making sense because it's 3.33am but hey, I tried.