The perfect explaination of why it is wrong.
When a child is involved with sex with a parent - it forever changes their views of what sex is. The same is true of adult/child sex - it forever changes the meaning of sex from a heterosexual experience for marriage and raising a family to the search for more young people that they can abuse for their pleasure.
Any people seeking deviancy will find it. In addition, they will bring about the ruin of society as has been displayed in other civilizations that became involved in deviancy, orgies, etc.
The problem is that our society has turned sex into a recreational activity, to be shared with whoever. It is no longer a sacred act between a husband and wife. So if our society continues to treat sex as a simply recreational activity then it is only a matter of time that people start to say, "Why must we deprive children of this fun?", which they are saying. And, if you take the religious aspects out of it then you could actually have an argument that it doesn't harm kids. As you said, it changes their view on sex, well if sex is just a recreational activity, same as riding bikes, then they will just hold that same view their whole life. And according to liberals, there is nothing wrong with that.