Well, there are two things that all this presumes which I feel aren't correct.
1. All porn is the same. No. There are a lot of subtile differences between the good and bad stuff. Good porn celebrates the human body as a work of art and/or the human sex act as a wonderful consumation of love. Love is consentual, and both parties are concerned about each other's pleasure. After the act, the partners continue to care about each other, share feelings, etc. Bad porn often is not consentual, has one partner dominating the other, neither partner is concerned about the other's pleasure and nowhere is love involved. Afterwards, one partner usually leaves the other, often in a state of emotional distress. Almost all BDSM stuff falls into the latter category.
2. Once a child sees porn, that child is automatically on the road to ruin. I can't buy that. First of all, should a child see porn, everything should be explained to him/her immediately, and the child should be allowed to express his/her feelings and thoughts about it. If a child knows that s/he shouldn't be doing this until s/he reaches the age of consent, the amount of damage is reduced. However, a lot of this depends on the child's level of emotional development. There are adults not ready to learn about sex, and five-year-olds who can handle the knowlege without any problem. However, parents tend not to be up front with children about things. The old thing about "the stork brought you" to cover up the truth can only make things worse later on. I was one of those children that was told "the stork brought you" until I was 11. I found some porn in the street. After getting a thurough whipping, I was told not to think about such things ever again, if I knew what was good for me.
A lot of good it did. I ended up a porn junkie anyway.