Is viewing pornography when you are already married wrong?
No matter how you view it porn is still evil in the eyes of God. We must resist temptation from the Devil in order not to fall in sin and stay in sin until you come to a point that it may be hard or too late to get out from the devil's snare.
Jesus said, "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart" (Matthew 5:28). The person watching an X-rated movie or viewing other forms of pornography is filling his heart with lust for someone to whom he is not married. Thus he is committing adultery in his heart. The probability is that when an opportunity arises to commit actual adultery in person, the person will not have much resistance since he has been filling his mind with these evil thoughts for so long that they have become a habit.