I believe everybody is bi, they just haven't been provided the room or opportunity to pursue these feelings. I've had an interesting sex life over the years, which includes 3somes, 4somes, and plenty of 1some's.
I never felt grossed out by sharing a woman with another man, even if we touched in the process...matter of a fact I found it incredibly intoxicating to have these experiences. So...If slapping a guy on the ass while he is blowing a load on a chick is bi, then I'm bi. I wouldn't gobble his dork or let him launch a rocket into my spacehole, but splitting a girl is fun and worth pursuing.