I'm primarily hetero myself, but I don't discount female options entirely, so it wouldn't be fair of me to say "no" to a bi guy. And if the relationship was somewhat open, it'd be awesome to have a MMF threesome (or FFM, though I like the former option better).
I had a classmate whose girlfriend was bi, and he was so paranoid that she'd cheat on him. I tried over and over to tell him that he was being unfair to her, but as far as he was concerned she was far more likely to cheat since she had more options. I don't get some people.
Especially since he turned right around and asked if he could watch me kiss the girl I was dating at the time.