You're not getting involved with another person, so I have no idea how people see it is a form of cheating. It shouldn't be considered as an offence in any way.
Whether they see it as a problem or not does not mean that watching porn is cheating.
Does that mean if they jack off to some image they created in their head, they are also cheating?
The OP may see it as a problem, but that is their problem and not their partners. Stopping your partner from watching porn in my opinion is just trying to be controlling.
It isn't cheating. I'd almost say that it's necessary.
Naturally, some people want sex more often than others. What are you going to do, in such a case? You're either going to have sex with your partner, who doesn't want sex, or deal with it yourself. Now, you tell me, which is worse?