errr, I'm surprised no one brought up the true point of this law yet.
The law was originally created because, well here's a scenario. Your parents are showing you porn from a young age, this can cause emotional/psychological damages. But how in anyway are they breaking a law?
It was made to keep children safe from things that could be harmful in the future. If police searched your house and found porn on your computer, they wouldn't care. Now if your parents had been making you watch porn etc. then they get a little more upset