At this writing, there is still controversy about the clinical label, “sex addict.” Some clinicians still argue no such thing exists. But to me, that doesn’t really matter, because if one partner is being hurt by the other’s lack of emotional or physical unfaithfulness, the relationship has a problem. No amount of denying the existence of a diagnosis will alleviate the heartbreak of the wounded partner. Therefore, if the “sex addict” wants to save the relationship, he or she needs to get help. Or the relationship will eventually end.
If your partner seeks sexual gratification outside your relationship, whether that comes through pornography or some other form of sexual activity, and if it hurts you enough that you question the relationship, I believe he and the relationship have a problem. And as long as you have a problem, the relationship will suffer, and in time it is likely to end.