Think of it this way...you're a guy..so follow me here.
Most guys, particularly younger guys like you and I don't really care to have too many female "friends". You're either ****ing the girl, or she's someone you'd like to **** if the opportunity presents itself. Now, I certainly know I wouldn't personally go out to a movie with any girl I didn't have some interest in ****ing. And this dude probably thinks something like you and I.
So, even if they're "just friends" in her book, he's setting himself up for some suck and **** action down the line. And it's that simple, man. It doesn't really matter what sort of intentions your girl may have at the moment, you know this guy "friend's" intention.
He wants to bang your girl, and if he hasn't already, then you're most certainly not throwing up any roadblocks for him by not setting up certain parameters in your relationship. Either get shit set straight really quick-like, or you can plan on your relationship falling apart promptly..if it hasn't already.