There are a couple threads like this but I don't have time to search for them atm. That being said, I think it's all a matter of what society pressures us into believing. Most guys have no issues with seeing 2 women making out/having sex or whatever but when it comes to 2 guys it's disgusting and wrong
I also think that alot of girls do it as a fad. It's the "in" thing to do so they makeout with other girls at clubs or whatever just for the attention.