That’s… Not a good argument. Child pornography and prostitution is illegal because it’s morally reprehensible, and incredibly, profoundly harmful to children. Same with murder, robbery, theft, etc. By definition, anything that is illegal is going to be done–or controlled–only by people that are criminals.
Does prohibition stop those things entirely? No, of course it doesn’t. But it gives society tools to fight against them in a way that decriminalizing does not.