is porn cheating?

Emily • Made of sunshine and lazy Sunday afternoons
Okay so I got into it with A girl on Facebook the other day because she said her fiancé was practically cheating on her by using porn to masturbate, and I said that it wasn't cheating. I personally don't see it as cheating, and think for a woman to tell her man what he can and can't be sexually aroused by is a little far, but what do you guys think?