Most Helpful Girl
There's a lot of biphobia, from both straight people and gay people. Straight people are biphobic for the same reason they're homophobic, gay people are biphobic because they might view bisexuals as "traitors" or unable to pick a side. Both straight and gay people often perceive bisexuals as being promiscuous, being unfaithful, just not having figured out their sexuality, stuff like that.
So basically it's the same reason straight people give for not wanting to date bisexuals, just minus the homophobic part. They're cheaters, they will be attracted to more people and thus be harder to keep around, they're just greedy, stuff like that.