Jesus said anyone who even looks at someone to lust after them has already commited adultery with them in their heart- this means you imagine being with that person you're looking at. But we have a different situation today- we have movies and music. Who don't imagine that the characters in the movie are themselves and their crush? Or music lyrics to be about your own personal relationship? (Stay with me).
A husband can watch a romantic scene and imagine that it's him and his wife. But he can't have sex with another woman and imagine she's his wife- because that is actual marital unfaithfulness. It is the same to imagine a scene in a play that you act in is an actual murder, and have it not be murder, but if you really do murder someone in the play scene, you are an actual murderer, or thief, or adulterer- if you are actually doing it in the scene, and not just pretending.
But nakedness is a whole other subject. There is only one relationship that shares absolutely everything with each other- a husband and wife. Only they should see each other naked in a sexual manner. Doctors might need to see you naked, a caregiver might, a statue or figurine might display some nakedness, but if there is no lust involved then it's not sin. We wear clothes because not everyone has that strength and maturity when they look upon someone without clothes. But to look upon porn (which is completely depleted of anything that comes from God- love, marriage, etc), is to join them in their sins that they are displaying on the screen.
Would I divorce my husband for watching porn? No, but there'd be a busted up computer and some major yelling going on! He would have to convince me he's extremely sorry and repent of that. When you marry someone, your hearts are completely open and vulnerable to each other. To hit someone in their most sensitive spot without warning, defense, or bracing themselves- that is totally the opposite of love. Love and trust once broken is hard to repair. Any spouse (husband or wife) who turns to porn instead of to the one they've vowed to love, is splashing mud all over the purity of marriage.
If they are involved with porn it is sinful and damaging to a marriage.
If they are imagining being with that actual person, or actually are with that actual person, it is cheating.