Not sure what category this experience fits into...you decide.
It was the weekend and 'James', 'Robert' and I were bored and had walked up to our Jr High and we skateboarded a bit until we were bored. While we walked around we saw a jacket that belonged to a girl that we really disliked laying out in the field and 'James' having been complaining about needing to piss decided it would be sweet revenge to pee on the jacket. Yes, very cruel I know but I was immature and went along with it as did 'Robert'. So the three of us surrounded the jacket and and got our penises out and released as much urine as each of our bladders could hold. Now I had never seen either 'James' or 'Robert's' penis and had never really given it much thought but while we were peeing on the jacket I could not help but give curiosity glances. I could see that 'James' was different he had an intact foreskin 'Robert' and I had been circumcised. By the time we were done peeing on the jacket I had an erection behind my shorts. So I decided to half jokingly make the suggestion that we should put cum all over the jacket. 'Robert' and 'James' laughed and then Robert turned towards the jacket and said watch this he pulled out his semi erect penis and worked it quickly into a full erection while 'James' and I watched and giggled. 'Robert' took about 2 minutes and ejaculated much of it flying into the air and landing on the urine soaked jacket. 'James' and I went down and looked at the semen close up and laughed. Then I got bold and took out my fully erect penis but did not get to do anything because we saw some people coming onto the field so I put my penis back while 'Robert' wiped his semen off with his shoe. We then went home and neither 'James' nor 'Robert' mentioned it the entire day but I sure thought about it. From about that time which was 7th and 8th grade for us until sophomore year in high school the three of us had about a many more encounters involving masturbation.