If you are thinking to yourself "that wouldn't bother me at all, that's fine," I would suggest that you are almost certainly lying to yourself. It hurts when your partner even thinks about having sex with other people, and this is amplified by conscious repetition of the habit. Am I inadequate socially? Sexually? Am I not manly enough? Almost certainly not, but it damn sure feels that way. From this understanding, we're only one empathetic leap away from understanding the emotional reason why porn is toxic to relationships. This, if you are in a committed, monogomous relationship that you care about, is sufficient reason to stop watching porn, all by itself.
But it's nothing compared to the reasons Gary Wilson articulates in his TEDx talk from several years ago, which extend far beyond relationships to your individual mental health and capabilities. I'll leave the explaining to the speaker and his 16-minute talk, but if 16 minutes feels a bit too long, and you're tempted to pass it up, I'll try to keep your interest by mentioning that your sexual capabilities, physiologically, may be at stake. Yes, erectile dysfunction from too much internet porn. "How!" you may ask? Here's the video: