Is looking at porn cheating?

There are two sides to this story.

Some arguments state that pornography forms a psychological idea of sexual expression which is harmful to the emotional and physical integrity of any relationship. It allows for the entrance into mind of sexual behavior outside of the relationship you are currently in.

Some people feel that when there is healthy sexuality within a relationship, pornography is the unnecessary expression of sexual desire without a relationship. They feel that it can be a perverse intrusion into the health, honesty, and meaning of loving commitment. Other people, on the other hand, feel that pornography can be a fun, healthy addition to a sexual relationship.

If your boyfriend, girlfriend, wife, husband, partner, or lover is engaging in ANY sexual activity, other than masturbation with ones self, outside of your relationship, you are the victim of infidelity. Monogamous sexuality is meant to be expressed between two loving individuals.

Some people may feel that pornography often leads to engagement in other sexual activities such as strip clubs (physical pornography/infidelity), cyber sex (with or without a camera; visual infidelity), phone sex (auditory infidelity), and eventually to intercourse with a third party.

A Christian view:
Jesus said (Mt 5:27-28) "You have heard that it was said to those of old, 'You shall not commit adultery.' But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart."
The purpose of porn, most often, is to arouse lust. So, yes, it is cheating.

Porn is very addictive and ruins marriages and relationships because a person who looks at porn a lot becomes a person who does not like to have real sex anymore, they would prefer to watch it, masturbate and that's all. This is very damaging to a marriage as you can see. A healthy relationship is based on affection between each other, feelings that arise from touching are indescribable. When you become addicted to porn you are missing all of this and you become a zombie with no inner feelings for anyone except yourself.

It is not harmless. It causes divorce & painful break-ups. The person who is watching porn gets addicted to it and they don't want to have anything to do with their live partner anymore. Yes, it does do harm.

On the other hand:
Most people consider pornography to be fun and harmless, and feel that masturbation is not cheating.

Some people feel that pornography can be kept in a healthy perspective within a relationship, and some relationships' sexual dynamic is even enhanced by the mutual enjoyment of pornography. In this viewpoint, a judgment on how healthy or unhealthy the viewing of pornography is depends entirely on the individual relationship and the individuals involved.

Others feel that while sexuality is something shared in an intimate relationship, it is still a very personal thing. This argument states that pornography can be used during private masturbation without necessarily affecting their relationship. These people may use pornography in moderation to provide sexual release.
These arguments assume "pornography" to mean media that depicts sexual activity between consenting adults.