A response to the original Gym Etiquette post brought new information. Something I was not aware of, and didn't think about. Plus, it comes from personal experience and knowledge of the Reader.
"Many reasons why you would find guys jerking off in the change rooms. Apart from it being much easier to get an erection after gym most of them will be doing that to take advantage of the anabolic window to get the muscles into recovery as fast as possible. Jerking off has benefits for that. You will even see a few just tugging away at it but never to ejaculate. That being said the anabolic window is different for every person. It can last upto an hour but its at its peak within the first 10-15mins after gym. Its not always with sexual intent. When younger and doing kickboxing our trainer always did it at the end of the session in the showers and he would encourage us to do the same just clean your mess up. We were the last group for the day and it was his own gym. He was married and his wife was a trainer too for the girls. He would be naked the moment he got in the changeroom. Talked with everyone with how their training was going completely nude, jerk off in the showers then close up for the day. It was awkward at first but after a while we were use to it and some of us would jerk off too. It depends on the environment and the people. - T"
So, the men who work out have either been taught this, or know it instinctually.