Been reading for a few weeks-this is my first story.
A few summers ago, I was at the water park I used to live by. I had a season pass, so I went pretty frequently. On this particular day, I had to work in the evening. This worked out well because the park was halfway between my house and work. I would just bring my clothes and some shampoo and shower and change at the park. I'd never taken a public shower before, so I was nervous.
I went into a change stall and took my suit off, wrapped my towel around me and got my soap and stuff. The shower was right across from the stall so I just walked across to it. I realized that the curtain wasn't the best fit for the shower, nor was it in the best condition. I couldn't keep the two openings on the side covered.
Shortly after I started showering, someone went in the change stall I came from. Aware of this and the curtain situation, I allowed it to just naturally pull aside instead of trying to stop it. I could see through the crack that the guy in the stall was peeking at my dick. I started slowly getting hard. It was hard to see through the crack in the door but I could pretty clearly tell that he was masturbating.
Regardless, he was in there for a long time. My dick kept getting hard and I had to fight the urge to tug on my equipment. I ended up giving a couple quick strokes but didn't cum or anything. To this day, I still think back to that day and I cum then though!