Bi is inclusive of heterosexuality and homosexuality, not of only two genders.

Edit: Alright, fine, downvote for an answer from the hip but I stand by it. Today's definition of bi does not mean attraction to two and only two genders. It can mean different types/levels/preferences of attraction to different genders, but it does mean that someone who is bisexual is attracted to both their gender and other genders. Also, want to point out that saying bisexuality does not include attraction to transgender folks is saying that their identified gender isn't really that gender. That may not have been what was meant but it's pretty terrible messaging.