Children for most of human history, grew up in one-room huts with their entire extended family, and saw it all. Not to mention that there was no age of consent for most of human existence. On the other hand, we don't actually have psychological profiles of all those hundreds of generations of kids, maybe they were all really messed up and didn't know it...

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort