Well, I learned to masturbate from a book–a book telling me about the horrors of it called God, Sex, and Me or similar. It even said it was okay to discuss it since anyone would discover it on their own anyways. I hadn’t. And it took me an hour of experimenting for it to happen the first time.
The reason I bring that up is that it didn’t go into cleanup, but it did mention wet dreams, and it discussed how, if you woke up with it, you didn’t have to clean up because your body would absorb it by morning.
So I was always under the impression that it would clean itself up.