I'm not sure I have an answer, so I'll just start by sounding out my thoughts.
Prohibition was very successful if you go by alcohol-related deaths during prohibition- the law saved many lives.
Prohibition was unsuccessful if you view only the rise of organized crime, enabled by a product many wanted that could not be legally sold. Still, keep in mind that many more lives were saved (less cirrohisis of liver deaths) than were lost to organized crime, Prohibition, as to saving lives, was a success.
So, banning a thing makes us consume less of it. Yet, banning a thing makes the lesser number of producers more violent, organized and criminal. An example of this is the problem with child porn rings and sex trafficking. Most of us civilized folk are surprised it exists - especially when it's in our backyards. But, when it does exist it is crazy violent and damaging to those involved.
Predictions- if porn was illegal, every porn star would be a doped-up slave. Far fewer people would view porn. There would be a lot less porn made and the little amount of porn made would be expensive…
But wait, porn isn't a consumable product in the way drugs or alcohol is. Porn that has already been made will stay in the market. Also, porn can be produced in the home… if the price of porn rose too much, it would even be made secretly by married couples that were really hard up for cash, so maybe I'm wrong about all porn stars having to be slaved, at a high enough price point, even normal people would risk it…
So, porn won't get -that- expensive, or people will just produce it at home. Under it's current legality, drugs, STDs and slavery still occurs- and it's harder to prosecute under the guise of legality. Further, the same people running the porn industry tend to have ties to organized crime anyway- there's sort of weird connections in the vice industry…
I'm totally uncertain but considering how bad the industry is anyway, and that it would certainly drive down consumption, I'm in favor of it's illegalization unless shown otherwise.