Because, first guys have hormones which give them the desire for sex? Correct? So in that case, we want pornography or sex. But when we watch pornography, there's a guy involved, and no guy wants to see a guy. So girl on girl is interesting because. Same with making out, I don't find any interest in a guy kissing a girl because I don't like the guy at all, but when girls kiss I can look at both. And its exotic.
Because most men have fantasized about having two women at once anyways so when they see to girls kiss it adds to that. Also it is a taboo that has been around for a long time and you will never see two straight guys kiss on the lips.
Because a guys attraction is a girl and when there are two involved it just makes it way better!
I think it may be visual ecstasy for the guy and they are hoping to have both maybe. I have a question for you and gals and guys too. What about it when a guy wants you to have sex with another guy and watch? My guy wants me to do this so I won't have an affair maybe or is it kinda weird?