You're not really addressing their point. Some people believe that drugs are "inherently harmful", but an outright ban or "war on drugs" made the problem worse. It pushed drugs to the unregulated black market.

The same would happen if you were to ban porn. Demand would still be there, people would just be forced to go onto the unregulated black market to satisfy their demands. Meaning it would make the situation much worse.

If porn were to remain legal than the government can resolve these issues by proposing regulation to mitigate the abuses.