my theory might be a little bit out there. but when a guy sees a guy and a hot chick having sex, he might be jealous of the guy, but when there's two hot chicks, and no alpha male, the guy doesn't feel threatened, and doesn't feel the need to protect his territory so to speak-so he can just enjoy the women, and not feel threatened by the guy.
as far as gay porn goes though, there was some study done that said that women enjoy looking at both male and female people if they are beautiful, while guys just like the females. i think that maybe when a chick sees two guys having sex though, that she can't see the beautiful women either- so it's not as enjoyable.
unity through nationalism isn't unity, its nazi. unity through shared pain, and human strugge- that's unity