https://en.wikipedia.org/wiki/Relationship_between_child_por... is a good starting link on this. When i last checked, there were maybe 5 studies total (imagine how hard it is to get those approved by the ethics committees), all of which found different results, some totally the opposite of each other.

Then again, it already seems clear that violent video games do not cause violence, and access to pornography does not increase sexual violence, so this case being the opposite would be unusual.