There have been forms of pornographic images for thousands of years. We see what we currently classify as "porn" enter a mass market to people of all ages with the advent of the "Playboy era." We all have that classic image of teenage boys hiding their dad's Playboys under their mattresses. Then came the internet, which allowed videographic porn to become an individualized experience for anyone with internet access. There are a plethora of websites that now offer free pictures and videos to fit every single taste and style possible. And they are continually replenishing and updating their content to keep everyone's developing interests satisfied.
I have been in relationships with people who have both watched porn and not watched porn. They all have ended, but that's not to say there wasn't a distinct difference in the two different sets of relationships. I have never dated someone who made me feel unloved. But, I have felt insufficient. In relationships where the person I was with consistently watched porn, I was incredibly self-conscious and jealous. I felt like I was just settled for since being with a pornstar was pretty much unrealistic. I felt like my presence was not enough, so another woman (multiple women) had to be brought in to satisfy whatever I couldn't in my boyfriend. I felt insufficient.
However, in relationships where my boyfriend didn't watch porn, things were just different. There will obviously always be jealousy and feelings of insufficiency in every relationship; that is unavoidable. In relationships where my boyfriend was not enamored with the pornstars of the internet, I was far less jealous and self-conscious, though. I did not feel as though I was a mediocre replacement to who they dreamt about. I did not feel like I was being replaced by someone else's face and body.
There is a multitude of studies that prove that unwelcome porn viewing in a relationship leads to unhealthy practices, negative feelings in the relationship or even breaking up or divorce.
Statistics like that lead me to believe that watching porn while in a relationship can definitely be cheating, depending on how you feel as a couple. Let alone, it can lead to "real" cheating where your partner ends up with someone else. However, I think saying that the person you're with having someone else in his head during intimate moments is not too far of a stretch to call it cheating.
If your partner chooses to ignore how you feel and the science behind porn in relationships, then maybe it's time to reconsider your partner. From my personal experience, I think unsolicited porn in a relationship is toxic and another form of cheating because we all know that you don't have to sleep with someone else for the emotional damage of cheating to be done. Relationships are hard; there's no need to allow fictitious people into them to make them harder.