That a bisexual person prefers one gender or the other doesn't "prove" they had any choice in being attracted to either gender, because that part happened without any input from them.
I didn't choose to be heterosexual, and I do not believe anyone can choose to be hetero, homo, or any other kind of orientation. All they can do is play the hand they were dealt, in whatever way works for them.
Some people may choose to 'experiment' with same sex, but that's not the same thing at all. If I decided to try having an affair with a woman, it wouldn't make me a lesbian, because I am not sexually attracted to women, and never have been. That doesn't mean I couldn't try it [or maybe even enjoy it], it just means there's a difference between a conscious choice and an innate orientation.
I could choose to have sex with a woman, but I can not choose to be sexually attracted to one - that's the difference.
OK next time I see the people that told me they made a choice I'll let them know they are lying. Glad you know better than they do. Simply because it doesn't fit your narritive doesn't make it false.
Sent from my Fisher Price ABC-123.