Just adding my experience...
I used to be absolutely fine about my ExH watching porn. In fact I was curious and quite turned on by the idea, and we used to watch it together.
As time went on, and the novelty of 'oooh, aren't we being naughty' wore off, I began to get bored with how formulaic most of it is, how bored and fake most of the 'actors' are, and how hateful a lot of the language is. Then I did some reading about the industry and got even more turned off by the whole thing.
Tried talking to exH about how I was feeling, and about some of the things I had learnt. Just couldn't get turned on watching a woman being 'willingly' abused knowing what her likely background was. ExH just got annoyed with me for trying to spoil it for him, and insisted on the 'Belle du Jour' fantasy that these are all just women who really love sex and isn't it great they're getting paid for doing what they love? His attitude to this really ate away at my respect for him.
He did eventually get a bit bored with it too, and stopped watching so much. But looking back I do feel our sex life was irreparably damaged by it. He always wanted to do things in bed that made me feel like I was participating in porn rather than making love to my husband. Things that did not turn me on and made me feel degraded, but which had become normalised within our relationship.
Once you see something, you can't un-see it. And the things we see, hear, do and talk about influence who we are. Having come to this realisation, I think it's better to give preference to cultural influences that make me feel good about myself and the world. I don't want that stuff in my head.
It wasn't the cause of the breakdown of our marriage. But his attitude towards women, sex and relationships was definitely a contributing factor. And I believe that porn was not a positive influence on his core beliefs about those.