The acceptability of viewing porn is subjective, you either agree with it or you don't. There are arguments for both sides but what matters is how you feel. If you don't like it then he should either respect that, or you should be more tolerant, or you're simply not right for each other. Nobody is right or wrong here. Everyone is entitled to their own fantasies and entitled to masturbate unless it detracts from your joint sex life. Where it would go too far is in forming personal connections with any of the models, such as trying to meet them or date them or speaking to them in a manner which is being unfaithful. I would also argue that subscribing to their content is a bit far, but that's not infidelity as such.
