I have often wondered the same thing. I'm not bi per-say just a little fuzzy on the lines and I hate labels. I am not against the idea of dating a women at some point but I also don't know many women and I am usually "one of the boys," so its not terribly likely to happen.
But regardless of that, I have this assumption that maybe bi-sexual people are just naturally a little more open minded. Like I said I don't know whether it is true but I have often wondered.
Another observation I have had is that there seem to be a large number of poly people who are gamers, (rpg's or mmo's), or who are members of alternative religions,(pagan or new age for example). Is this because these things require a person to think outside of the box or is there some other underlying commonality?