Gym rat here. I go to the gym 5-6 times a week. Been a gym nut for years. Have never seen any sign of sex, masturbation, or even dudes cruising. Most people who go the gym are there to better themselves and most of the time are lost in their own thoughts as they work on themselves. The closest thing to what you described I have seen was when I was spotting a guy and that guy spotted me. We talked, went to the locker room to change together while we kept on talking, and somehow I ended up in his apartment after we left the gym. Ended up dating that guy for over a year. But this was many years ago. And before we left the gym together we never even mentioned anything sexual.
move to NYC or LA or any major city gym ---you will see it in any steam or sauna---and some really hot young dudes doing it---but not in stalls.
The porno where 2 dudes saw each other in the locker room and just started having sex is pure fantasy. Don't believe that actually happens in real life.