Several years ago when I was in college I ended up living in a house with three guys for awhile. I was friends with one of the guys, but not involved with him. The other guys were friends of his.
I grew up living with just my mother, in a pretty sheltered environment, so I never knew how horny guys were. These guys jacked off all the time. I never had any mutual sessions with any of them, but at one time of another, I walked in on or caught each one in the act at least once.
The first time was when I walked by my friend, John's, room. I heard something funny from inside. I knocked on the door, called his name and stepped in. He was totally naked on the bed, with his hard, oily cock in hand and a Playboy magazine laying beside him. We were both shocked. I apologized and backed out. I was shocked, but pretty turned on about it.
Later, I apologized again, but he told me not to worry about it. then I asked him why he was doing it, when he had a girlfriend. He told me that he just got horny a lot, and liked to jack off when she was not around. He told me that his girlfriend worried about getting pregnant, so mostly they masturbated each other when they were together. He also told me that while they didn't talk about it, the other guys jacked off a lot too.
The other guys were not home, so he took me into each of their rooms. In Tim's closet, was a stack of about 40 girlie magazines. He told me that he and Dave (the other roommate) borrowed these from time to time. We laughed, knowing what he meant by that.
The first time I saw Dave, leave Tim's room, with a handful of magazines, he had the guiltiest look on his face. I told him it was OK, I knew what he did with them. He tried to deny it for a minute, but I said 'Don't try to tell me that you're not going to go into your room and jack off. I know that you are. It's OK.' He mumbled something, and went to his room. A few minutes later, I listened at his door. I could hear the squishy stroking and pages turning. I had to go masturbate myself after this.
This happened all the time. I found remnants of semen a lot. I love that guys get so horny and have to beat off. I wish I had been a little bolder back then, and actually watched them stroke off. There is nothing hotter than watching a naked guy jack off and spew cum.