I read your reasoning on "Is Porn Cheating?". Even though its self gratification, looking at another man/woman in a certain way/situation(sexually) other than your companion is still cheating.
This was not discussed in your article. The need to look at another individual to satisfy your needs is cheating. Then to make it worse, your companion does not know about it.
If the couple watches porn together, they are engaging in sexual gratification with each other and enjoying each other at the same time. Now if one partner does not want to watch porn with the other, then that's another issue. Why can't they??