I was 14 at the time of this story and had only just discovered masturbation. One of my favorite things to do was to go into public restrooms and jack off.
One day when I was at one of my sister's swim meets I felt like jacking off. So, I headed over to the public restroom. On entering, I noticed that it was a one stall bathroom, and that the stall was occupied. I walked over and stood outside of it, waiting to enter. I couldn't help but notice the large crack between the door and the side of the stall. I positioned myself for the best view of the inside. I watched for a few minutes as a boy about the same age I was as he struggled with a bowel movement. The moans and grunts, along with his ever-reddening face was giving me an extremely large erection. Finally the boy finished and as he stood up his small, hairless cock bounced around. He finished up and exited the stall.
I immediately walked in and turned around to lock the stall door. But there wasn't one there. While normally, being a young boy, I would have backed out being to afraid of being caught, I was too aroused to be too concerned. I shut the door and pulled down my pants, followed by my small white briefs that had been tight around my boner the entire time. I sat down on the toilet, still warm from the boy before and began savagely attacking my erection. My penis wasn't much larger than the boy's before me but it was at full length. I closed my eyes and began remembering the sights I had just seen. Just as I had faded off into the realm of fantasy, I heard the stall door being yanked open. I looked up in shock only to see a man about 40. I sat there, boner in hand, white briefs around my ankles, turning red. He quickly shut the door, mumbling something, and turning red himself. As soon as the door was shut I made about three strokes and the end of my penis exploded with cum. I cleaned up and exited the stall, forced to walk right past the man who was waiting to use the stall, the same man that had caught me.