I like this thread because it's gotten serious and it's turned into an adult discussion. In my opinion, these are the best threads.
Before I add my bit I'd like to commend @Tranquilitybasehotelandcasino whose earlier interventions made it worthwhile for me reading till the end.
What I want to say is two things (and first of all make the disclaimer I'm a man):
The objection that porn is abuse and demeans women only holds if what you're watching is actually porn. If you watch clips from mainstream movies with sexy scenes in it, whether on YouTube! or on Pornhub or whatever, that wasn't originally porn. The viewer might be using it as porn, but it's not porn. They may be movies that were shown the world over and won multiple awards. Or even when they weren't, movies with good production, known good actors (who not only do sexy movies), and good directors. I'm not naming names but you know there are.
With the question of when someone's using some non-porn material as porn deserving perhaps its own chapter, and I don't claim I know the answer or I can elaborate fully here, but when would that be? Someone going into a mainstream cinema and watching a movie they know may contain scenes that'll turn him or her on? Someone going into said cinema without telling their partner? Someone watching a movie for the second time? Someone buying the DVD or saving it to the computer, or saving the web bookmark? Someone using the bookmark when they're on their own, or, a step further, when they're on their own for their own recreation or leisure, or further, for their own private recreation + physical pleasure? Where do you draw the line? You see there's going to be as many answers to be as there will be readers (posters). But I said I wouldn't answer this question.
And my second point: To those who say watching porn (real or substitute porn) is cheating, I'd ask this: Isn't a central element in the concept of cheating that there's a time order, or arrival order, between the person or object you're cheating with, and the person who's allegedly being cheated on? That the person being cheated on can claim that he or she was first (either because she was known to the would-be cheater first, or because they've made an 'agreement' first), while the person or object you're cheating with is allegedly second, so they came later in time? And that time ordering and claiming to be first is what allows the person being cheated on to assert their privilege, to say "I am the cheated-on one".
But what do you say to this: What if a person is seeking, for instance, solace or arousal in fantasizing about same sex relationships, or else heterosexual sex but of a kind or nature their partner isn't in a position to provide to them, AND the tendencies of this person predate their meeting their partner, AND it wasn't disclosed at the time, or asked for, or if asked for the person preferred to keep that private? So going back to those tendencies or preferences was never in the marital agreement or contract or whatever, because it wasn't explicitly agreed, only implied (and we all know what happens when contracts are ambiguous enough or flexible enough that even those who subscribe to them have a different understanding what they mean).
But isn't in this case the idea of cheating turned upside down? E.g., a person with secret same-sex preferences could assert that his secret fantasies predate their partner, so giving in to their fantasies in private isn't cheating on their partner, it is actually being with a partner that's 'cheating' on their fantasies (who got there first, as in, they came into his or her life first) and it's actually the partner that's second in line and does not align (even though their partner might not know about it). Or use any example of fantasies that's not same-sex but the partner can't provide; it doesn't matter. There is no need to make any given fantasy a central element - which could be different for everyone - only the time ordering, i.e., fantasy predates partner (so please don't linger on any particular example; after all, it is something that everybody else does not know about the person who's watching porn; it is a question mark).
And the person who's allegedly cheating could assure you at some point they had to choose between remaining single while keeping their fantasies, but be alone (suppose that it wasn't actually possible to live their fantasies), or entering into a relationship while not completely letting go of their fantasies, and for them the latter was preferable and even they're happy with the choice of partner they made. Except they were not happy that their partner claiming precedence or the right to know or to do a deep inspection of their mind (into their long-held secrets), or into their private time (including alone time), or into personal conduct (having to give account of what they have or haven't done, or thought of, or will do or commit to do).
You can see the person who's allegedly cheating (by watching porn or substitute porn) in this case has a defense too, and it goes through asserting independence and freedom and the right to one's privacy and the right to be themselves.
So I am not convinced that the answer to this is clear-cut. I think there is only so much one can enquire on with respect of a partner, no matter how close they are or how much they love each other (or not). I think one has to accept there're things that will never be known, or understood, and conducts that one's never going to be able to witness or be able to control. And personally I think that's a good thing. I think partnership is about union, but not fusion, and it's about voluntary disclosure, but not compelled disclosure. Otherwise we'd be abusing the alleged cheater in the same way we do not want women (or the weak one in a relationship) to be abused (understanding for that, controlled, coerced, or worse). I think giving someone the right to be themselves (even for a fleeting moment out of time, out of sight), can't possibly be a bad thing.
And let me end by saying, for those who may say I'm making it too complicated, and this is only about watching big arses and boobs on Pornhub so it's really very simple: I say I'm not talking about those. Since the matter is simple (allegedly) I''m happy to take a step back and not cover that. I'm only talking about 'complex' (deep-mind, ingrained?) cases. Because really I can't pass judgement about 'simple' cases. If I refuse to call cheating that of a complex case, who am I to say it is clear and to pass judgement in a case seemingly simple. It seems to me that'd be discriminatory and I've not the right to do that. So I'm not condemning those other ones either. I simply do not pronounce myself about them one way or the other.