I was at the gym one day and I was getting out of the whirlpool and the physical trainer told me to go ahead and change and he closed the curtain. I was dying to take off my bathing suit because I had got excited and had a hard-on I wanted to let free.
I took my shorts off and released my hard seven inch dick. I was getting ready to change into my jeans, and get home to rub one off, when the trainer must have realised he forgot something and barged in. I was naked with a hard cock.
He stared at it at first and then pulled his sweat pants down slowly, exposing a hardening cock in his tiny sport briefs. I slowly went over to him and touched his growing dick through his briefs. The head was quite large and the length seemed bigger than mine. I pulled the front of his briefs down and his large hard cock popped out. He then sat down on a bench and, remaining standing in front of him, I bent down and began to slowly jack his cock while he reached out to me and gave me a nice gentle hand job.
After a while he jerked me off a lot harder. 'I'm gonna cum!' I whispered in his ear and he sat back and I sprayed my cum on his chest.
To this day, following my work-out with him he stays in the room and watches me change while he beats off, always cumming as I walk out the door.