I realize these posts are a bit old, but I thought I'd add a personal experience where I was "caught" (sort of) in the gym showers.
It was after a good work-out, a swim, and some time in the steam bath. I had headed back to the locker room to shower and change, and normally I would be with a group of guy friends, but my gym buddy wanted to stay and go swimming again, so I headed to the showers alone.
When I got to the showers, there was this middle-aged guy there who I had seen around the gym a lot (mainly in the sauna and the showers). He always seemed to blatantly sneak peeks at me, and this made me feel a bit uncomfortable as I entered the showers on this day, because I got all nervous and thought that he would probably be checking me out in the shower as well.
Well, when I entered the showers (and these showers are group showers in a big open room), I immediately took my towel off and hung it nearby since I always shower naked. The other gentleman was showering with his swimming trunks on, but when he saw me naked, he proceded to remove his shorts as well. At this point another guy came into the showers, probably early 30s with a toned body, and also proceded to shower naked on the other side of me.
The middle-aged man was pretty much just washing his cock and balls at this point and looking over at me, and getting all embarrassed, I started to get hard and was sporting a semi-hardon for the majority of the shower.
I think the 30-something guy saw as well, but kept showering as though this wasn't a big deal...though I think it started to get the middle-aged guy excited. Both guys were better hung than me, so my semi-hardon made me look more their size, but you could definitely tell I was aroused.
I finished up my shower, toweled off in the shower room, then wrapped the towel around me and went to the lockers. I didn't make an effort to dress quickly, but after I left the gym, I wondered if the same situation would happen again. It hasn't, but one never knows.
I've never seen another guy fully hard in the showers, nor have I seen anyone jerk off in the showers or in the sauna...but there's a first time for everything I guess