I think one possibility has been completely ignored by everyone. After 10, 15 or even 5 years sometimes - many things have changed between two people. When people completely lose interet in their partner and seek stimulation by p 0 r n, it's not always so cut and dried. Often central to this situation is that one of them, or both of them do not have anything to share with each other, yet they are too afraid to end the relationship because they're so used to it and can't imagine not having it. Yet there are aspects of themselves emotionally, physically and mentally they would like to share with others that are not their partners - yet they can't so they seek outlet with p 0 r n, or developing a "harmless" online relationship.
It isn't always that they don't love themselves or are afraid of intimacy - they are afraid of leaving something that is no longer intimate.