Here's a thought: Would you rather your kid look at porn and masturbate or go have sex? One or the other will happen and yes sometimes both will happen but I can assure you that one will.
Any way I agree with the first reply, the best method is to have open communication and trust with your child so you both don't need to hide things.
All he is saying is he'd rather them do things under his/her care so they know their kids is being safe about it rather then some where else where the adult doesn't care therefore the child is in dangers way. There is no actual comparison between porn and heroin.
Technically, it is their business to see what you do since you know, raised you, and gave birth to you. Why not watch porn and not masturbate so you don't have an urge for sex in general since you are not practicing those "muscles", self discipline is needed. From a religious stand point you are pretty retarded with some of the people on this thread, but that is my opinion since I am looking at this from a religious perspective.
Anyway, for keeping an eye on your children my dad used to keep logs of what I used to look up. If your son/daughter does anything you don't like you can talk to him/her. If you do this they will understand that it is wrong and hopefully get them to stop and understand why it is wrong. My dad used to block things like that but I got past it like a normal kid would do, but once he sat down with me and started explaining it was wrong I stopped doing it, I was embarrassed because of that so that's why I basically stopped.