The only assumption I have is that the shower masturbation was added with a DLC since it wasn't in the base game. I know I saw it happen. There was no girl walking up to me, in fact, I was actively avoiding any that weren't showering because getting caught means they get angry unless I get out immediately and clearing angry states reduces favorability, which can then lead to a break up. I specifically interacted with the shower door as is meant to happen when peeping. Like I said, I'll get a screenshot once my main PC is running again.
I'm aware of that page, I use it all the time when trying to maintain horny state on each girl, though that has gotten a lot easier with the trainer since all I have to do is make sure it locks onto the target girl's data before I end H scenes or talk to them again and use the trainer while in conversation. I do still occasionally try to maintain the horny state the way it is meant to be done, though, and that's where that page comes in handy.