It's not hindsight at all. MM told them about the sexual sounds that night. He told them about Sandusky and a boy in a shower. He was visibly upset. That could see/hear all of that for themselves yet they decided to go tell a football coach about the incident instead of the police. Where the hell is the common sense in that? Why aren't you upset about that?
You keep saying he heard sexual sounds. He heard 3 slapping noises and his mind concluded it was sexuals sounds. That does not mean it was sexual sounds. That's why Dr Dranov asked him if he actually saw anything happening to which MM replied no. I know it's not easy but, try to put yourself in Dranov's shoes. So MM says he's outside the locker room and hears slapping noise and his mind says they sound like someone having sex. So what does MM do? He walks into the locker room. Would you just walk into a room where you thought someone was having sex? I know I wouldn't but, that's just me.
After he walks into the locker room, he peeks into a mirror trying to catch a glimpse of the two people having sex. We know he's a bit of a perv so that's understandable. Probably trying to check out some naked chick but instead of seeing that, he sees Jerry and a kid. Doesn't actually see them doing anything but that was what he was expecting so it freaks him out. Dranov is just trying to piece together what exactly MM is telling him and after asking questions comes to the conclusion that nothing MM is telling him warrants a call to the police or children and youth. Still thinks it's a bit weird that Jerry takes his 2nd Mile kids to the Lasch building so he suggests that Mike tell Joe about it. That is not an unreasonable conclusion to make based on what MM told him.