Ninety percent of college-age guys masturbate, and the other ten percent lie about it. Unless they have some medical problem, men that age get horny a number of times every day, and unless they have an especially compliant and convenient partner they need a release from the tension. Dorm rules need to recognize this.
Some will insist on privacy to jerk off, so toilet areas should have enough stalls to accommodate that as well as their excretory function. (Potty parity freaks, consider this!) But for the majority of dorm residents, masturbation in the room with the roommate present should be stated as a normal and acceptable practice. If the roommate wants to participate, so much the better. When matching up roommates, this should be one of the questions asked.