I had a roommate do that one time back when I was in college. I transferred to another dorm room right after that, didn't give him any explanation. I ended up sharing a room with some stinky, bipolar guy that I caught standing over my bed and staring at me while he thought I was asleep. I moved out right after that happened too. Third time was a charm and I had decent roommates after that (although there were some minor issues).
I don't know, it would have just been weird to have to have a conversation telling my first roommate to keep from jacking off while I'm six feet away from him. I was a lot younger then though. These days I would have told the guy I'm a very light sleeper, and if I catch you wacking it around me I'm going to go tell the dorm manager and request you be kicked out.