Fun topic. Here's a true story; I haven't thought about this in several years ...
One of my most memorable experiences was in a rather old (but nicely redone) gym locker room downtown. There were eight showers, I think -- four on one side, three on the other, and one at the end. There wasn't tons of lighting. Only the stall at the end (the "handicapped" one) had a (white) curtain. That stall also had a frosted window, which got lots of midday sun.
I was grabbing a quick workout out on a gorgeous summer holiday weekday morning -- maybe Memorial Day, as I think back on it. So it was pretty empty, since it was a downtown location. I had seen a guy and his teen son working out, but didn't think much of it. I end up in the locker room with them just as dad is heading to the showers, leaving son behind. It's pretty clear he didn't want his dad to see his junk. Or me for that matter. My locker is right next to his, and he kept lingering. I was fine with stripping quickly to give the guy some privacy.
I get back to the showers. Dad is showering away uninhibited, and there's another guy in the stall closest to the handicapped stall, but sort of across from it. I take the stall next to that guy. I can't see him, but we both have good views of the handicapped stall.
I'm not surprised to see the son enter the handicapped stall in his towel. I am surprised how well I can see movement behind the curtain, though. The combination of the white curtain, window, dimly-lit shower area ,and summertime sun was creating a back-lit, silhouette type of effect that I had never noticed before -- mainly because I was rarely there at that hour.
Son hangs his towel on the hook behind the curtain, turns on the shower, and begins cleaning up. His silhouette is plainly visible. His genitalia is average-sized, if that. It's hard not to see him rushing through washing his upper body, hair, and legs. He stands in front of the spray and I see his penis get a bit larger. He's not touching it at first, but he's definitely on the verge of getting an erection.
Then he starts washing his genitals. This quickly evolves into stroking -- slowly, deliberately. He steps closer to the curtain, outside the spray, and sort of leans over the handicapped bench that is right at the shower stall's entrance. His left hand is against the wall, and his right hand is now in full jackoff mode. Again, he's average-sized, if that.
That's when I see the guy in the stall next to me take a step outside his stall. He's watching this whole show, and is jacking slowly. The son is almost furious in his masturbation. He's standing right over the little bench when I see the tell-tale signs of a guy ejaculating -- the pause, the resumption, more pause, etc.
The guy in the stall next to me is fully hard when he sees me. He's very embarrassed, finishes showering and wraps himself in a towel, which was tenting pretty badly.
I watch the son dab at his hair and body as his penis returns toward flaccid. He's not dried off but nevertheless wraps the towel around his waist, opens the curtain, and walks by. His towel is tenting slightly.
By the time I get back to the lockers, he's made sure he's already in his cargo shorts so I don't see his private parts. Dad is nowhere to be seen. The son quickly puts on his shirt -- oblivious to the fact that not only had I seen his privates, but that I had seen him in his most private moment -- and heads to the sinks to tend to his curly and wet hair. I see the other guy emerge from a toilet stall in just his towel, likely fresh from his own ejaculation.
I end up walking out of the locker room right behind the son. Dad is waiting outside. He asks what took him so long. Son just ignores the question. Dad puts his arm around his son, who maybe three minutes earlier was ejaculating in the shower. As we walk toward our cars. I heard the dad say they had just enough time to get to the movie.
Chances are good that dad knew exactly why it had taken his son so long ...