When I was about 14, there was a group of kids in my street aged between 11 and 15 who all played together, boys and girls. One day, we had been 'exploring' in an abandoned, overgrown builders' yard, and I found myself with two of the younger girls, and needing to take a leak.
I excused myself and turned to pee against a wall when I realised they were watching me with curiosity. On impulse I turned to let them see my penis. They laughed and commented how fat it was. The attention caused it to stiffen immediately and it rapidly began to rise. They drew nearer to get a closer look and to assist them, I pulled my cock and balls clear of my pants. As I did so, my pubic hair came into view.
I remember one of the girls saying, 'Oh look, it's sprouting hairs!'. I was so aroused by then that without thinking I started to masturbate. The girls watched with fascination, not knowing what I was doing or what to expect. Within a minute or two I shot several thick jets of sperm in their direction. After I'd explained about masturbation, they asked me to do it again!