nidur
Do your same sex friend's sexually arouse you? Can you chose not to be physically attracted to them? If so you might be straight.
If you so firmly believe that your sexual preference is a choice then choose. Choose to be gay. Choose to go down to your local gay bar. Choose to be sexually attracted to a woman. Choose to take her home and maintain physical arousal long enough to reach orgasm with her. Choose to become emotionally connect to her as your lover. If you have a choice you can prove that you you are bisexual.
Unlike being a robber of a glutton or a sanctimonious judgmental cretin, sexual preferrence has nothing to do with action. It is what you prefer. Much like you would prefer to eat or prefer to breathe, homosexuals prefer the company of their own gender. You could certainly fast or hold your breath but it doesn't change what you are and more than a homosexual or bisexual living in the closet can pretend to be something they aren't.