This is no longer a hypothetical. There are actual psychologist and doctors that are stating that some forms of adult-child sex result not only in no harm for the child, but perhaps as a benefit. So if religion cannot be brought into the discussion how are we to combat this?
As far as why parents would do this, I don't know what is the issue in these cases but many people are afflicted with pedophile thoughts that they cannot control. Usually brought on through their upbringing. Now if their mind is feeling that sex with a child is truly a way of portraying love for that child, what do we tell them to prove to them that they are wrong? This is the same with homosexuals. They think that since they FEEL a certain way, it excuses them to act a certain way. And without using God, how do you oppose their views?
This is why a Godless law system can never work.
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.