I love chicken parm. I hate eggplant parm. Eggplant parm makes me nauseous. Am I making a choice that I hate eggplant parm? No. I could make a choice to eat eggplant parm (or have gay sex) and possibly vomit in the process, yes. But no reasonable person would do that.
Are we cleared up now?
It seems like you're agreeing that it IS a choice, but are also saying some people are so set in certain of their physical preferences in mates (such as gender) to the extent that they would not reasonably choose anything other than their personal preference.
If that's what you are saying, I agree with that assessment - everyone evaluates different things when making choices, including sexual preferences, and there's nothing wrong with that.