Was your sexuality a choice? Obviously not. It is no different for bi people.
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.
Bisexuality definitely wasn't a choice for me and from talking with a lot of men and women who I met, even in ex-gay groups, it wasn't a choice for them either. I think that is probably the result of biology and childhood experiences (speaking only from my own experience).
The internal feelings of attraction towards both men and women? No, that's not a choice. I can remember having attractions to other girls as young as about 7 or 8 years old. Of course at the time it wasn't exactly "sexual," but the idea was there of other girls being pretty, nice, etc. It wasn't until I was much older that I was able to put a name to it.
What you do with those feelings is the choice, just like anything else you do, as in take action on, is a choice. I can feel excited about Christmas morning but yet choose not to scream at the top of my lungs. I can be downright furious and choose not to punch someone in the face. I can be hungry and choose not to eat an entire box of doughnuts.
In the case of my sexuality, I chose not to date women. But does that mean I have chosen not to be bisexual? No. I am still very much attracted to women - that is not a choice.
Being bisexual is not a choice. The feelings you have, the attraction in your head it happens whether you want it or not.