used to happen to me a lot in my younger days. Just kept the towel on, then I finally got my head around it and it never happened again. Never see a guy with a boner at my gym but find the younger guys never let the towel go. It is funny to watch them struggle to pull up their boxers without the towel falling. I also found a wank before the gym never worked, unless it is in the shower.
Don't think about it and it won't happen