At the 2003 American Academy of Matrimonial Lawyers meeting, two-thirds of lawyers reported that compulsive Internet use played a significant role in divorces over that year, and 56 percent of those divorce cases included a partner who had an obsessive interest in pornographic websites.
i.e. one of the partners was using porn as a band-aid to ease the pain of a much deeper problem -- which typically takes one of 3 forms: (1) both partners love each other and are doing "everything right" but one has discovered, before the other it would seem, that they just aren't really compatible sexually; (2) they're still hot for each other, but other distractions (fighting, money, kids) are getting in the way; or (3) one of the other parts has "discovered" that they have a different chromosomal preference.
Nasty and potentially intractable issues, indeed. But porn watching by itself -- even excessively so? In comparison, not so much.