Deviant sexual arousal can be argued to be deemed unnatural, although it is so common as to come within the range of normal! Many couples use pornography within their exclusive and private intimate relationship, and with consent.
Personally, I abhor pornography, whether it is 'soft' porn or hardcore, but I acknowledge that others have differing tastes and incorporate pornography into their lives routinely.
The issue for the OP is that one half of a couple has been found to be using it without the other's knowledge and consent. They have some talking to do, if they can.