Many of the people that think being gay is a choice are in fact bisexual. So for them it’s a choice on whether or not to act on it. They assume everyone else works the same way.
I choose to be monogamous with my partner. When I was single there was not a single conscious thought rattling around the horn brain.
I would think that everyone choosing their sexuality implies that everyone essentially has the ability to ‘be’ any sexuality.
From a logical perspective that implication is true, choosing your sexuality implies you have a choice. However, I disagree with the premise that there is a choice to begin with.
I do not choose to be straight, I just am. I’m not gay, and no amount of choosing will change that. I’m a guy, and I can choose to look for a boyfriend, but it won’t change the fact that I’m attracted to women. Now maybe I discover that I’m actually attracted to both men and women, but I would argue that discovering is different from choosing. Choosing would mean that I can choose to not be attracted to a gender, which I can’t. I can only choose whether or not I act on it.