I imagine it's hard to study since you need to find two countries with similar socio-economic status, rights of women, attitudes towards marriage, rape etc but that have opposite opinions on pornography.
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".