I don't think just because he uses porn automatically means that the next step is unfaithfulness. It could be but people who have never viewed porn before have affairs, so I don't see a correlation. Further, some would argue, including myself, that his behavior is already unfaithful to you, even without the actual physical act of sex.
Usually when a guy looks for outlets like these it is either a) they feel lonely or loved, b) they aren't getting something from their relationship marriage, or c) they are just a womanizer.
My guess is that c is not the case of your husband. When my wife was pregnant with our daughter sex was non-existent. I started getting a bit of a wandering eye wondering what various women would be like. Would they find me attractive? Would we have the physical chemistry I lacked and craved with my wife? I never cheated but if the right situation had presented itself who knows?
So is there anything that you aren't providing that he might be needing? Is his turn to porn at all possibly the result of something you could be doing better in the marriage? Only you can answer that. Unfortunately now he has discovered this outlet and it sounds like he has developed an addiction.
#1 I would try to convince him to get help. #2 I would get and read Dr. Phil's Relationship Rescue. #3 I would start implementing the changes to yourself that Dr. Phil outlines in order to see if just by those changes you can get things back on the right course.
Good luck. Remember, you won't change your husband by brow-beating him, he has to come to a realization that what he is doing is unhealthy and want to change. You can possible encourage that by being the best spouse that you can be.
That is the realization I made a few weeks ago. I had gone about my life and marriage blaming my wife for our relationship ills. Finally I looked in the mirror and saw the problem staring back at me. I can't change her, only she can change her. I can only change myself for the better and hope that my changes inspire changes in her.
