it doesn't matter whether or not porn is or isn't infidelity...all that matters is the event of one person not liking it when their significant other indulges...
many a porn-viewer will tell you it's just for fun, means nothing, etc -- as such it should be easy enough to forego in the greater interest of maintaining respect and trust with one's other half...
most of us have given up relatively insignificant habits and indulgences for the greater good of our relationships...porn-viewing, for some, falls in the same category as flirting and/or staring at others...
there is no right or wrong, only preferences...

those who think porn is fine no doubt have preferences of their own that those who don't like porn might find petty and reflective of an insecurity...

this could well be question #3,095 for the dating questionnaire: i don't like porn at all...if you currently view it, will you stop for me?