Is using porn in a committed relationship a form of cheating

I think so. So does this author:

"By “cheating” I mean that using porn is breaking a vow—either implicitly or explicitly—made to one’s spouse. This is because marriage is, in part, about sexual exclusivity; it is about “forsaking all others.”

This is why so many women say using porn feels like cheating: the act of seeking out another woman for sexual pleasure—even if she is hidden behind a veil of pixels and a sleazy acting agency—is not a movement towards faithfulness, but away from it.

... people have been blinded by the sense of distance the digital world places between ourselves and the real world. We believe something doesn’t count as much if it is “online” or “on television” or “just fantasy.” We rename offenses: stealing becomes downloading, cruelty becomes speaking one’s mind, and exploitation becomes entertainment. We have settled for what Chris Hedges calls an empire of illusion. “Pornography does not promote sex, if one defines sex as a shared act between two partners. It promotes masturbation,” Hedges writes. “It promotes the solitary auto-arousal that precludes intimacy and love. Pornography is about getting yourself off at someone else’s expense.”

Yes, Using Porn Is Cheating. Here's Why.