There isn’t anything that farfetched about this scene from Str8 to Gay. These days, more and more women are admitting that they’d be turned on to watch their male partners fool around with another guy. You can say what you will about that type of arrangement, but it’s not too radically different from a straight dude getting aroused by two chicks together. Duh.
That is, it makes sense until you focus on Johnny’s girlfriend, who sits there fully-clothed, hand under her chin, acting like someone who’s trying to stay awake during a boring college lecture. She is such a creepster! And I’m kind of obsessed with her.