I have witnessed dozens of encounters at my gym over the years. The first one was the Sports Connection in West Hollywood, then Crunch.
Mostly it's guys jacking off together. Once in the steam room there were four or five guys in there and one guy just leaned over and went down on the dude next to him and sucked until he came while the others just watched. I think we were all a bit surprised because there was nothing going at that point. No one left or said anything. When the guy giving head left, we all looked at each other and laughed. The guy who got the BJ said something like "I've never seen him before, but i didn't want to be rude".
A guy in scrubs (he likely worked at the nearby hospital) came into the shower room and was obviously high on something. He just undressed and climbed into the shower stall with me. I asked him what the hell he was doing and pushed him out, but he got back in and grabbed my ass and pushed his crotch into mine then tried to kiss me. I pushed him out again and he stood there for a minute staring and stroking. He was very high, so i just laughed it off. It's a good thing he was hot, otherwise i might have been upset.