Was your sexuality a choice? Obviously not! It is no different for bisexuals.
For me its not a choice, it is simply who I am. Its been a part of me my whole life.
Bisexuality isn't a choice. It's the word for what happens when you go through puberty and start having "funny" feelings about people, the same as a straight person's first crush, or a gay/lesbian's first crush.
Involuntary feelings of arousal and teenage discomfort around sexy girls flooded me when I was in gym class, the same as if I were a boy in the girls' locker room. I felt very out of place and confused. Like anything else that happens to us growing up, the experience happens first and then the explanation.
I didn't know what bisexuality was until I was in college. I had already been bisexual for four years without knowing it, because no one can choose how their sexual feelings emerge. It's like puberty. You don't choose to go into puberty. It just happens.