(age 27) Another triumph of communication, but I don't think you needed to ask him to leave you alone so you could masturbate. I was never in a dorm in college, but I've occasionally stayed in them for retreats, conferences, youth hostels, etc.
As you state, everyone knows that both he and his roommate masturbate in the dorm room, so all you really had to do was ask to be alone and he would have understood what you were going to do. I would make a practice of never talking with the guys in my room or even make eye contact because I knew I was going to masturbate and I didn't want any of them confronting me about it if they heard anything.
I just figured it would be harder for them to say anything if that was the first thing they had ever said to me.
I also chose the bed (whenever I had a choice) based on which one had the most privacy to masturbate. Most guys don't say anything about masturbating to the one guy they live with for a whole school year in a tiny room.
Thus, the easiest solution to the problem of masturbating in the dorm is to discuss it with roommates and lay out ground rules.
Unfortunately, many 18-year-olds find it hard to acknowledge out loud that they masturbate.
We were both cool about it, and left each other to finish in privacy.
Many other times, I heard him masturbating when he thought I was asleep.
I wouldn't have gotten to experience that at that point if I hadn't said anything.
(age 23) Obviously, it would have put no strain on your relationship if you had recognized aloud what you were doing.
As a rule, guys just wait to masturbate until their roomie is asleep.
I'm going to college next year, and I'm wondering what guys do in dorms to masturbate.
Do they jack off quietly in bed or do they do it together at the same time?