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.
this threads pretty hilarious, the only similar experience I have is on a school camp this weird kid who got put in our room woke us all up moaning and talking in his sleep about another girl in our class, some of us got up to see if he was awake trying to be funny or something but he was asleep with an obvious boner just moaning and say oh stephanie for like a half hour while we pissed ourselves laughing. Next day he denied it