"Can I have it both ways? Can I eat lunch, maybe sit together on a dorm couch and watch a movie, with some other guy… and then return to my boyfriend in the summer?"

Hell to the no. Disregard what all the other "college females" are doing. Just because everyone else is doing it doesn't make it right. I thought they taught you that in middle school. That's inappropriate behavior in a relationship and if you really cared for him you wouldn't consider doing shit like that. If you really want to start dating other people, break up with him. Don't mess around with other guys behind his back. He sounds like a nice guy and deserves better than to be treated like that.