It's impossible to answer here, as many people (a) have a heterosexual partner who could leave them by saying it (even if they're bi) or worse, (b) have a heterosexual partner is a serious LTR reading this who will. I honestly, think it's like 1/3 gay, 1/3 bi, 1/3 straight, but that's just an educated guess. If there was a way to put fully anonymous surveys here and fully anonymous posts, you might get something closer, but people get paranoid over that, having IP address tracking and whatnot. Maybe one day when people don't get so freaked out over this, we'll know, but given all that, and the fact that many people lie to themselves, it's impossible to know right now.