Your first passage about criminalizing art is 100% correct and 100% irrelevant. You cannot call porn art. Porn with adults, children, dogs, pumpkins - all that stuff is made for people to get off, not enjoy the emotions that real art provokes in people. Therefore we cannot compare criminalizing porn with criminalizing art.

There are edge cases, of course, when art might be provocative and considered immoral, and maybe even illegal sometimes. But that would be edge cases, highly debated.