My take on this phenomenon is that until relatively recently most men would never imagine that a homosexual would be found among them - that is if they even thought of gay people at all. Then as gay people became more or less mainstreamed in society since the 1990s, I think straight guys realized that any random guy next to him could be checking him out, and it made him feel uncomfortable. So he started changed under a towel. From there, I think that undressing under a towel grew somewhat virally, until it became the norm. In addition, gyms got rid of gang showers (which also cut down on cruising) in favor of stalls, and even replaced curtains with doors. High schools probably feel showers are a legal liability because it may create opportunities sexual harassment. (We all remember that one teacher that found an excuse to take a stroll through the boys locker room.) Today, most gyms do not even offer single sex hot tubs. I think that single sex steam rooms are next to go.
