Without taking a stance on the question of child sexuality - what you say is true, but is there any particular selection pressure for it to be off, either? Evolution goes for the simplest solution, and "always on" seems to me simpler than "off until a specific age, then on".
Of course, that's an oversimplification. The required machinery may simply not be developed yet, in the same way that you need to first grow to be four feet tall before you can grow to be five feet tall. But then, when you reach the size of four feet, you already have four fifths of your five feet-tallness in place, so it stands to reason that that at least part of what makes sex pleasurable will be in place before adolescence. Whether it's active is obviously a separate question, but I don't think "has no particular evolutionary reason to be active" tells us much by itself.
Anecdotal: Approximately 30% of the material on Quizilla et al. Whether they're writing/reading about it solely because they think it's adult and edgy is a different matter, but there are clearly many children thinking about this kind of thing at the very least.
What do you believe that most people on this site don't?
I'm especially looking for things that you wouldn't even mention if someone wasn't explicitly asking for them. Stuff you're not even comfortable writing under your own name. Making a one-shot account here is very easy, go ahead and do that if you don't want to tarnish your image.
I think a big problem with a "community" dedicated to being less wrong is that it will make people more concerned about APPEARING less wrong. The biggest part of my intellectual journey so far has been the acquisition of new and startling knowledge, and that knowledge doesn't seem likely to turn up here in the conditions that currently exist.
So please, tell me the crazy things you're otherwise afraid to say. I want to know them, because they might be true.