technically, its not. but if ur partner says they dont want u watching porn while ur in a relationship with them, then that's cheating. if they don't mind, then its not.
In the end ur using someone else's body for ur viewing pleasure other than ur partner. 100% cheating, if ur partner doesn't mind they don't care about you.
Now this is a GREAT time for this question: do you think its cheating to watch porn when you’re in a relationship?