I actually dealt with this in my last relationship. Not with porn (I rarely look at it), but with strip clubs.
Now, it's not that i have an obsession with strip clubs or anything. I work with a bunch of younger guys on a sales floor and we make a lot of money. We go to strip clubs a lot. In fact for our Christmas party we went to a strip club and my boss gave us money for tipping the girls and we had an open bar and all! It's just how we roll!
Well, I got with a girl who had a big problem with strip clubs. She was worried I'd have sex with one of the girls (she was a pretty jealous girl overall). I tried to assure her I don't go to strip clubs to try to have sex with the women. In fact, I see those women as objects. I have no emotional interest in them. I simply go to have a good time with my friends, which I consider every last one of my co-workers to be.
She couldn't accept it and the conversation always ended with her saying, "I just don't want to talk about this anymore." This whole situation showed me a lot about her that I didn't like, e.g. lack of trust for me, lack of communication skills, etc., and it, amongst other things, was pretty much the reason we didn't last.