I think this video I posted earlier may answer that--this one and the Scary Close videos:
I don't mean to sound demanding

.... but I think you should watch this TedTalk.....the main point he's making is that we all have an innate desire to bond with others. When people haven't developed their relational skills.....they end up resisting that genuine relational bond (out of fear of exposing themselves) and go for the distortions that still give them the same brain high (like porn and flirting online--even dating). Once things become all "unveiled" and there's no more ways to hide and present a facade (like living with another person.....being married)....I think (I'm not an expert) there's more fear in the marriage and a desire to gravitate towards what's familiar and given that high (back to the porn.....back to the flirting....back to the presenting a false image of self and getting flattering responses). Like the other video from Donald Miller implies, though, people that hide themselves from others never get to experience genuine love--and they keep chasing something they're preventing. This isn't a marital issue.....it really is an individual issue that's just become full-blown *because* of marriage.
I wondered the same thing along the lines that Zoo asked......is he looking up violent porn?
For yourself.....since the actual change is up to him....I'd suggest learning about healthy boundaries. Dr Henry Cloud (if you use FB) has excellent insight and advice:
Dr Henry Cloud (video) .....and book
The Power of the Other