Many people don't consider it "just porn". It's real nudity (if images) and real sex (if video).
There are many people who consider porn to be cheating and they have a right to feel that way. I will add to that though is they should discuss their belief with their partner before they get to any stage of commitment in a relationship (moving in together, engagement,etc).
If people consider porn wrong, that's their choice. Finding out your partner has a porn addiction is no different from finding out your partner has a gambling addiction or a drug addiction - some people are ok with heavy gambling and some people are ok with drug use, just as some are ok or ambivalent about porn use but women shouldn't be forced to ignore how they feel if they are not ok with those things if they consider them wrong.
My first husband had a drug addiction and I got sick of people telling me to just be gentle with him about it because there is nothing wrong with illicit drug use. Even when it got to the point where he was facing jail and I had to go begging to charities to feed my daughter when he'd blown all our money on drugs. Sometimes you have to stand up for what you believe in.
If a person believes something is wrong, they have a right to say so. You just need to find a loving gentle way to say so - being assertive not aggressive but not passive either.
My second husband has struggle with a porn problem, we both believe it is wrong but we are totally open about his struggles - you can believe a behaviour is wrong and still be completely open about it. It just requires gentleness. You can be completely mature without downplaying how wrong a behaviour is.
If I found out he was looking at porn again (and hadn't told me), he'd have no problem with me gently voicing his behaviour was unacceptable - the key is in doing it gently - assertive but without any aggression.