I'm not sure about the porn. I think it's direspectful if one partner has a bunch of videos and naked pictures of the other partner to use instead of porn and yet still chooses to use porn. That's pretty rude. But I think the actual watching of porn isn't cheating. Because people can dream about another person or fantasize and then in that case everyone would cheat weekly. But IMO I'd have to say anything that you can't tell your partner and that you feel you need to hide so u won't get caught is most likely a form of cheating. So if someone's watching porn and hiding or lieing then I'd consider it cheating for sure because its leading to being unfaithful and trust issues will develop. Had this problem with a serious ex of mine. Never got resolved because he never came clean or kept his word an I never felt good about him using porn so excessively and hiding it when it wudnt of been a problem of he had been honest and when he had my material to use instead.