Why do guys get turned on when two girls male out?

Guys like girls, the shape of the bodies and softness of there lips. All things about them really. But guys do not like the shape of other guys. They dont get anything out of a guy and girl making out because the guy and girl still has a guy in the equation and a girl cant make out with herself. so therefore guys like it when the thing they like to see makes out with the other thing he likes to see.