This phrase ‘locker room behavior’ is used to suggest that the rules of socially acceptable behavior are relaxed in the locker rooms used by the people who use such rooms. The phrase is sometimes invoked to try and excuse atrocious speech and behavior, most infamously by Donald Trump when during the 2016 campaign he was caught on tape boasting about his sexual assaults on women.
I have never been part of any organized sports program and so am not familiar with locker room culture at all. But recently I went to a tennis club to watch a match and before I left to go home I went to the men’s bathroom. The bathroom turned out to be part of a locker room and I was a little startled to find a couple of old guys about my age wandering around naked. It is not that nudity bothers me but I am just not used to people I do not know wandering around casually like that. When I later mentioned this to a young man who is familiar with locker rooms, he commented that he had noticed that it is older men who seem to do this.
I recalled his comments when I recently came across this Zits comic strip, which suggests that this may be quite widespread.