Recently, I read an article about someone whose parents would purposely have sex in front of him when he was a young child. Many of the comments left in response to the article remarked that this amounts to child abuse. (For a less extreme example, it's commonly held that exposing young children to porn or graphic sex scenes is similarly inappropriate.) I agree that this sort of thing is egregious, but I don't know how to explain why. When the child is watching his parents have sex, what exactly is happening that harms him?