>There are those who claim to be "bisexual": they'll choose a man sometimes, and a women other times.
Erm no they don't. As a bisexual person let me school you: I don't "choose a man sometimes" and "choose a woman" other times - at ALL times I'm attracted to men and at the same time to women.
Who I choose to sleep with on this occasion is determined by availability, individual attraction and circumstance - but it's not a choice about WHAT I'm attracted to, I am ALWAYS attracted to BOTH.
Bisexual is not something distinct from gay or straight - which are not THAT distinct from each other. Sexual orientation is a spectrum and nobody is entirely at EITHER end. The closer you are to the middle the more bisexual you will identify.
> That ability to change indicates a choice
No it doesn't, you have no evidence to back that up and the testimony of most people who make such changes do not support your assertion: the vast majority would say they were lying before that point, usually out of fear of oppression.
Even if you could factor those out - the remaining few may not indicate a choice at all - you yourself gave one reason why not: hormone changes.
Sexual orientation is a physical attribute of our bodies (including our brains) - like all other physical attributes it can change over time - but that doesn't constitute a "choice" anymore than you CHOOSE to have your hair turn gray or your scalp go bald when you get older.