Some great points made here, MXM. I think another reader described it well when they said that the point is perhaps phrased strongly, for effect -- and perhaps because it is a bit quippy, doesn't point to or account for the differences in relationship dynamics from one couple to the next. One couple's unforgiveable is another couple's Saturday night, depending on the context and experiences of the individuals involved, not to mention cultural and social mores.

Your model is a thoughtful one -- relationships are as diverse and complex as the people that comprise them!