His argument is invalid.
It seems to rest on the idea that porn is a single, unitary industry run by some kind of single, unitary authority, and that therefore if you condone or consume any part of it you are somehow responsible for all of it. Even if the porn you condone or consume is morally OK and victimless you are then supposedly duty bound to answer spurious questions about how much harmful, violent porn is "justified" by it.
Quite simply, if someone watches a piece of non-violent, consensual porn, then they are responsible for their OWN actions not anyone else's. If someone else somewhere in the world is watching someone being raped at the same time, and you disapprove on the basis of it being rape, then that has nothing to do with the other person. We're not morally responsible for the actions of other people doing other things.
In fact the writer gave the game away when he admitted that he doesn't actually accept that any porn can be OK. So, as is often the case with these things, his aim isn't really to make the argument he pretends to make. His true aim is simply to assert that all porn is bad. He would have been better off admitting that at the beginning and not clouding the issue with a poor argument attempting to show that "good" porn doesn't justify "bad" porn (which noone ever claimed it does, and there's really no reason why it should).
It's a bit like people who hold ordinary peaceful muslims responsible for the actions of Al Qaeda, and expect them to explain and justify how they can carry on being muslims when that makes them "part" of something so terrible. When you dig a little deeper into such "arguments", you invariably find someone that just doesn't like muslims.