Earlier quoted context omitted.
"how to avoid a larger part of it" not true though; most of the existing porn will remain where it currently is. New sites might choose to start up with .xxx, existing big sites might buy the .xxx counterpart and keep their .com. The end result is no reduction in non-.xxx porn, and some extra porn on .xxx. Totally pointless.
The other thing is, most responsible adult companies already work to make sure their sites are labeled clearly as adult-only sites. Parents have options to prevent their children from going to these sites. In fact, these sites are safer for children than Facebook when you get right down to it.
Usually the idea is to keep it away from children because children will willfully look at it without understanding. That page that says "Absolutely DO NOT look at this if you're under 18" has no relevance to an under-18 year old.
It's the same reason we put all other restraints on teenagers; if they were capable of detecting trouble and reliably refusing to participate therein, then they wouldn't need all of the things that are in place to protect them from themselves, foremost among these being parents.