The easiest way to tell might be to compare cultures where normal pornography is easy to get, to those where it is very difficult to get, and see if the rates of sexual attacks and deviant acts vary between the cultures. Does anyone know if such a study has been done?
Comparing different cultures with each other doesn't work, you can't determine weather differences are due to the availability of pornography or to a wide range of other cultural factors.
What you do is compare a single culture with itself, before and after a major change in the availability and content-range of porn. In fact a substantial number of such studies have been done, across a substantial number of countries. The results are consistent. Increases in the availability and content-range of pornography are generally followed by a decrease in rape and other sex crimes, or at worst no change in those rates. This result also extends to a smaller number of country-cases that included child pornography becoming legal. In every such case rape, other sex crimes, and child molestation always decreased. Countries where child pornography changed from legal-to-illegal had increases in child molestation rates.
A Google Scholar search can turn up a variety of such studies. Here are links to one two of them.
The Danish liberalization of legal prosecution and of laws concerning pornography and the ensuing high availability of such materials present a unique opportunity of testing hypotheses concerning the relationship between pornography and sex offenses. It is shown that concurrently with the increasing availability of pornography there was a significant decrease in the number of sex offenses registered by the police in Copenhagen. On the basis of various investigations, including a survey of public attitudes and studies of the police, it was established that at least in one type of offense (child molestation) the decrease represents a real reduction in the number of offenses committed. Various factors suggest that the availability of pornography was the direct cause of this decrease.
Pornography continues to be a contentious matter with those on the one side arguing it detrimental to society while others argue it is pleasurable to many and a feature of free speech. The advent of the Internet with the ready availability of sexually explicit materials thereon particularly has seemed to raise questions of its influence. Following the effects of a new law in the Czech Republic that allowed pornography to a society previously having forbidden it allowed us to monitor the change in sex related crime that followed the change. As found in all other countries in which the phenomenon has been studied, rape and other sex crimes did not increase. Of particular note is that this country, like Denmark and Japan, had a prolonged interval during which possession of child pornography was not illegal and, like those other countries, showed a significant decrease in the incidence of child sex abuse.
I wonder what the world would look like if we had legislators who legislated on the basis of evidence and reality rather than ideologies and soundbites.