My first experience with this.
This happened during high school when I was about 16. It was summer vacation, a time for relaxing and hanging out with friends.
One day me and my best friend were chilling at his house. In his basement he had a pool table and a nice area to hang out. So we got to shooting some pool. I don't really remember who was winning, but we were kind of just fooling around and kept messing each other up.
He kept hitting my hands with the pool cue to mess up my shot, so I said to him jokingly that I would pants him if he didn't stop. I thought that would work since he probably didn't want to get pantsed. Well, he did it again so I made a half hearted lunge at him.
To my surprise though he reacted by pulling my shorts down to my ankles, but leaving my boxers up. I decided to try and get him back for this, but he got me again, this time pulling my shorts and boxers about halfway across my butt.
Being a teenager, I began to get an erection, but I didn't want him to see this so I tried to hide it as I tackled him to the floor. We wrestled around for a few minutes before I got hold of his belt loops on his jeans. Laughing I told him that I had him now. However, he grabbed for my shorts and tried to grab the elastic band as I tried to wiggle away.
His hands brushed across my stiff penis and his fingers moved up and down the shaft, though he couldn't see it. He asked me if that was my penis. At first I didn't say anything, worried what he might think. But as his fingers continued to walk the shaft I admitted to him that it was.
To my surprise he just pulled my shorts down a bit and began to rub my penis. I thought that this was what masturbation was like, but wasn't sure.
After a couple of minutes of this I stood up so I could pull down his jeans and see his. It was the first penis I had seen that was not my own. His was a bit longer than mine, but mine was a little thicker. We both were skinny and mostly hairless except for we each had small tufts of pubic hair.
We stood there for a little while each rubbing the others penis. Then we decided to play pool where each shot let the other person have to do a dare. After a few shots I was dared to pull my pants down for 15 seconds, but as I did this he just walked over and we began again. We did this for a little while longer before I had to go home.