I imagine this is something that has been studied extensively, what good/bad o outcomes are known from societies that do/don't allow porn?
It is ok to not have to differentiate them, since mostly you are after a correlation between the different underlaying attitudes.
The banning of porn is a symptom - it is a sign of the attitudes, and you can EASILY test against that.
Such things have been done. I remember a study discussed on HN where they used the same neighbourhoods before/after a change of law. That wasn't porn though, I think it was strip clubs, and the result was something like "when a strip club opens in an area that had none before, the number of sexual assaults in the vicinity decreases by a few per cent".