I was shy about masturbation, but my best friend wasn't.
When I was 12, my best friend revealed himself to be totally uninhibited about masturbation. We had frequent sleepovers, and one Saturday morning we were on the couch in his family room watching cartoons. I was on one end of the couch and he was on the other. We were both wearing just t-shirts and boxers, as that's how we slept.
Suddenly I noticed that he was fondling himself through his boxers, and I could tell he had a boner. I was really surprised, and interested. He did that for a while, then pulled his dick out of the front slit and played with it in full sight. His was about the same as mine; about four inches, slim, and uncircumcised. He was mostly just rolling his foreskin back and forth while watching the TV, almost as though I wasn't there. He did this for quite a long time, then started jacking off.
He didn't seem to mind that anyone walking in would have been able to see what he was doing. After a few minutes of concentrated stroking, he stopped, slid down so he was in a more reclined position, and slid his boxers down to his thighs, then resumed jacking. He tensed, held still, and shot several small jets onto his stomach, then just lay there with his eyes closed.
After he relaxed, he turned to me and asked, 'don't you want to do it, too? It feels good.'
I answered, 'I know it feels good, but I don't feel like doing it right now.' The truth was that I did, but I was too shy about masturbation to do it in front of him.
So he said, 'In that case, could you go get me a tissue to clean up with?'
I said 'OK,' and did. He cleaned up, pulled his boxers back up, and resumed watching TV as if nothing had happened. I waited until I got home to do it, and all I could think of while jacking off was of watching him do it.
That became a weekly pattern, and he also began doing it in bed at night right beside me. He seemed to have no modesty, and frankly, I really enjoyed watching him.
One Saturday morning he was doing it when his older brother (I think 15) walked in. I was surprised when my friend didn't stop, but kept going. His brother just said, 'You going at it again?' and sat down in an upholstered chair. He watched for a minute, then he pulled his own shorts down and started doing it in front of me. There I was watching the two of them go at it, so I finally gave in and decided to do it, too. I pulled my boxers down and jacked off, watching the two of them go to it. I was really turned on, and came really well. His brother had a lot more spunk than either of us. And guess what, I got the usual job of fetching the tissues (for some reason none of us ever thought to do it in advance). This time, though, I had a stomach all covered with semen, so it dribbled down and got my boxers all wet and sticky, so after we cleaned up, I went and borrowed a pair of my friend's. From then on I remembered to get tissues in advance, and we continued to do this every time I slept over. His brother joined us many times.