Earlier quoted context omitted.
You can't expect parents to hover over their children 24/7. Apply the argument you're making to any other instance of corrupting minors. If you have sex with a 14 year old, you're not going to be able to say, "Well the kid's parents should have been watching him..." to the judge. The same applies to showing porn to one.
This is a straw man. Saying that the burden is on parents to ensure a child's safety doesn't mean a child molester is free from blame. You've taken his point and pushed it to the extreme.
The commenter above is putting the burden on the parents in order to claim that a person who shows porn to children is free of blame. How is that a different argument?