I was 14 and a high-school freshman. It was fall and football season. I had just come back to my locker from the showers. I had nothing but a towel wrapped around my waist.
A big, athletic boy on our varsity team had a locker directly below mine. He was undressing and his locker was open, so I sat down on the bench between the rows of lockers and waited for him to finish. As he took off his gear, my curiosity began to grow, until he stood with his back to me in nothing but his jock and his socks. Looking directly at his hairy, muscular butt, I became powerfully aroused. I hid my boner with my towel as best I could.
He slid his jock down, and turned towards me, putting one foot after the other on the bench beside to me to take his socks off. I eyeballed his package, but I tried not to stare obviously at his dick hanging right next to me, almost in my face.
Then he turned back to his locker and bent over to put his jock and his socks in it. His legs were spread apart, and I could see the back of his scrotum dangling through his legs and the head of his dick below his balls. As he bent over, his butt-cheeks parted just enough to give me, sitting directly behind him, a brief glimpse of his asshole.
He closed his locker and strode off to the showers. I sat there for a few minutes trying to get control over my hard dick. But it was hopeless. Somehow, though, I managed to get dressed without attracting attention from other boys in the locker room.
I headed immediately to the john. There were no doors on the stalls, probably to prevent boys from doing exactly what I needed to do. But I took the stall furthest from the door and managed to release myself without getting caught. I masturbated compulsively several more times that day, until my dick was too sore to do it any more.
I've never actually had a same-sex experience. But the image of that boy bent over butt-naked in front of me is seared forever in my consciousness. My masturbation fantasies often revert to that incident, and in my fertile imagination, we've done many things.