Okay, ever heard of someone who bi, turning gay or straight?
If you're bisexual then you can be with both. You can choose to live a gay relationship or a straight relationship.
Being gay is different though because they only like the same sex, just as a straight person only likes the opposite.
You can't help what you're attracted to. You can't pick that. Could you easily feel what you feel towards a girl to a guy if choosing was that simple?
You can say it's a choice in philosophical terms but in emotional terms (which matter), it's not.