For her: Masterbating / sex lowers sex drive(for the few hours or day that you did it). So she is kind of 'cheating' us out of better sex. She would enjoy it more, and so would I because I would know I was pleasing her more, if she wasn't watching porn/masterbating.
I know from experience as a man, that if I masterbate a lot, say Monday I am really horny, and I end up masterbating 3 times! If I do so, sex monday would SUCK, and last like 5minutes because I wouldn't be able to stay hard because I had already came 3 times today. Tuesday sex would be ok, I would stay hard and sex would be good, but not feel as good as it could. I realize females don't have to get 'hard' during sex, but they must still have the sexual build up, and she is wasting it on porn - I guess that is what bothers me.