We all go through the blame stage. As long as eventually you come out of it with healthy knowledge about how you can improve yourself rather than endless beating yourself up with "what if's" then you'll be fine. I've not quite made it yet, so I understand it's easy to say but difficult to do. I think we all are 50% of creating the situation which allowed the problem/affair etc to occur, whatever it may be, but in the end only the individual can choose to act. Him choosing to puruse the online dating was a choice he made.
Have you talked to him about your feelings? Pornography is something that many people accept in their lives and it may be just that you are coming from different viewpoints and need to understand each other's view and compromise. The dating is different beast however.
Reality is that which, if you don't believe in it, doesn't go away.
