Marriage is work, no matter how you look at it. That being said, if something hurts the other partner it is wrong, no matter what it is. For a small ex: Say your husband tells a joke that offends you. It stops being a joke, when it offends or hurts the other person. You can apply porn or anything else to this.
Personally, I think if a man looks at porn, he is not sitting there just looking, more is going on through his head and/or body and mind. I would ask, "If God came for you while you were watching porn, would you feel you would be ready to meet him?" [Confused] I am sorry, but I am up front and honest in what I believe. I did not mean to offend anyone though!