My son has a best friend who is always around, staying over, coming on vacations with the family, etc..
I have caught my son beating off a couple times and he me, so we're pretty open about it and don't mind seeing one another.
At our house we get most of the movies channels like HBO, Cinemax, etc so there is always some porn to watch at night. My son usually watches these every weekend and sometimes I'll come down to join him but usually I let him be. One night I heard talking coming from downstairs, I just thought he was talking to himself while he was going at it, so I went downstairs and saw he was on the phone. I went into the other room and got on the other line of the phone. It was his friend who's always around. I knew he and his friend were close but I dind't think they were this close to where they masturbate together.
So, I guess his friend gets the same channels because they both were saying things like 'oh my god, this is awesome' and 'I'm so hard right now' stuff like that. I just continued to listen and soon enough they were both moaning into the phone. This got me pretty hard and excited so I deciced to join in on the fun, I dind't say anything I just stroked my cock as I listened.
The conversation went on for about 10 more minutes and then I heard his friend say he was going to come and my son told him to do it. His friend started moaning and breathing very heavily and I could hear his powerful orgasm. Soon after the friend it was my son's turn and he began panting and moaing just like his buddy. After they both had came they continued talking for awhile about their orgasms and the movie then said goodnight. I hung up the phone and then walked into the room as if I hadn't heard anything. I came in and said 'good one tonight?' he said VERY! and that he had already came. I asked if he'd mind if I watched it with him and he agreed as usual so I sat down and after about 5 mins of watching another scene came so I pulled out my cock and started stroking it with him right next to me. He'd watch and say stuff like, oh c'mon dad you can do better, just joking around as we often do. after a couple minutes I shot 5 or 6 big ropes of cum all over my chest, stomach, and hands. We sat there a bit longer talking and then called it a night.
It was a lot of fun hearing my son and his friend having such fun and then joining my son after. I'll proabbly keep my ear out at night from now on to join in again.