I have to say it was almost by accident. I was used to watch boys like in the showers, but when I saw men, real men in full display, all muscles, and body hair, and the deep voice while they were talking and making jokes, and their c*cks, hairy, uncut, all that bush of hair around...I was caught with my eyes fixed.
It took me a while to realize it, when called for military duty, back in the days barracks had common showers and the senior waited until midnight to spend more time in hot water. Couple of weeks I needed a late shower and they allowed me. That was the moment.