Although one typically thinks of male predators, it can work the other way too. From another post, let me change the genders and see how it reads:
I guess I'm going to go against the grain here and not totally vilify the man. Mind you, I am NOT saying what he did is right - of course a man should not be in the girl's locker room. But I can totally see how this situation could happen. Your daughter says she left her goggles in the locker room. You tell her to go get them. She goes in and then takes waaay longer than you think you should. What do you do? Now, if he had thought a bit better, he could have flagged a female down and asked them to please go into the locker room and find his daughter. But what if there wasn't someone to flag down? I'm really not trying to be argumentative, but at some point - if my kid isn't coming out of the locker room- I'm going to open the door and yell for her. What other option do I have? Yes, he absolutely shouldn't have stepped in. But I could see someone doing that out of frustration with their kid.
Again, I'm not saying that he was right, but maybe people should cut him a little slack. It gets tough when your daughter is too old to accompany you to the men's room but still young enough that you worry.
If he repeatedly does this than there is an issue that needs to be addressed, but a one time thing? I personally don't think so.
Unless there is very good reason to believe that the child is in danger, not just taking annoyingly long to find their goggles, there is no reason for an adult to enter a children's locker room and particularly one for children of the opposite gender.