Ah, gotcha. See when you mentioned the strap on, I thought that it was intended for use by the women to the guy. =x Probably because that's a curiosity of mine. I don't know if I'd call it an outright fantasy, but I'm definitely curious. Now I'll admit, the other part of the appeal of a two guy threesome for me is that like a lot of men with lesbians and bisexual chicks, I find the idea of watching two guys make out or do other stuff to each other to be really freaking hot. Would I expect that out of the two guys? No, but the thought that it was a possibility would sure as hell be in the back of my mind and it'd make the whole situation that much sexier.
I can kind of understand why you wouldn't want competitiveness in sex, but it could be fun depending on how you look at it. Now if you're going to let it affect your "fragile male ego" then yeah it wouldn't be fun. But if you treated it like a contest and took turns trying to see who could pleasure the girl the most, I'd think that would be kind of fun. Especially for the girl.
Seriously though, I wouldn't mind doing something like that with a female friend of mine if the situation was reversed.