I am a christian, not bisexual, but if we are to believe that God forgives all sins, think of all of the other things we do that are deemed sins (not that I see being bisexual as a sin, because I don't). If he forgives one, he forgives them all, because he doesn't differentiate how "bad" one sin is compared to another.
Jesus died on the cross to forgive us of all of our sins, so you are good to go.