This happened to me when I was around twelve. In the small town where I grew up, every boy attended the Saturday Kiddy matinee at the Rialto Theater. We gathered in a mob in front of the theater waiting for the doors to open.
On this fateful morning, we settled into our seats as the movie came on. We noticed the dirty old man, wearing a wrinkled suit move down the middle aisle to sit in the center of a row of seats near the front. Kids instinctively moved away from him. Halfway through the movie my friend sitting next to me nudged me and nodded towards the old man. In the dim light I didn't know what I was supposed to be looking at. The old man sat with his hands in his lap staring at the screen. Then I noticed one of his hands was moving. He had his fingers wrapped around a huge erection. He was playing with himself.
Every kid in the theater was watching the old gent. They began moving closer till they surrounded him, to get a better view. They didn't dare even breathe lest the old man know they were watching him. The man unbuckled his pants and pushed them down to his ankles. He stroked his massive hardon fully knowing he was being spied on. Every boy in the theater wanted to join the old man. The old man stroked faster, slapping his meat filled fist against his naked thighs. Every boy knew that sound and was turned on by it. A few squeezed their steel hard dicks in their pants. Many were oozing precum.
Suddenly the old man stiffened his body and moaned loudly. He jerked his meat with a frenzy till a thick, long, ropy stream of milky cum spurted high into the air. Two more streams followed, all falling back onto his naked hairy belly. Evey boy squirmed and moaned. Some grabbed their crotches unable to hold back their orgasms. Those who were old enough soiled their pants. The others experienced dry orgasms. The old man pulled on his trousers, then without a word wiped his spent tool with a paper towel and pushed it back into his pants and exited the theater.
I made my way out of the theater to a rusted hulk of an abandoned auto body hidden in the middle of a field grown high with reedy grass, my safe haven for playing with myself, where I got naked to spend the rest of the day, till dark, beating my meat and cumming four times. Every Saturday after that we looked for the old man but he never returned. The matinee was never the same for those who were there that morning. After that, the manager caught many guys playing with themselves during the movie.