Yes, pornography is cheating, because it involves deception and dishonesty. It can hurt a relationship because it is a kind of affair. It involves an object or image instead of a person, but it is still an affair.
Pornography can become an obsession. If watching it is an occasional behavior, it might not have any effect on a relationship. But it can be a very powerful thing that people quickly develop a problem with. It becomes similar to any other addiction.
When you become addicted, you start lying. The behavior can make you lose interest in your spouse and in many of the the activities you used to enjoy.
If pornography has become an addiction for you, you have to decide: is it more important than your relationship? If you want to save your marriage or relationship, then STOP. If you can’t stop on your own, get help. A therapist can help you figure out if the behavior is filling a need in your life that isn’t being met in another way.
If it’s your partner who is addicted, you may feel hurt or betrayed. You may be furious. You might feel ugly or inadequate. (Why am I not enough?)
You’ll need to tell your partner honestly how you feel. Try to resolve the hurt and to rebuild trust. Be open to seeing a therapist who can help you explore whether other problems in your relationship might be triggering the need.
Some people are able to stop when they realize the harm they’re doing. Accept, though, that your partner might not want to stop, or might not be able to change without treatment. There are 12-step programs available to help, for example http://saa-recovery.org/.
Don’t let a partner’s use of pornography make you feel bad about yourself.