Okay, I remember going to a wedding back in the ’70s where a couple read vows they’d written themselves, making clear that they weren’t sexually exclusive. And yet, not too many years later, she was royally unhappy with his philandering. Was that unfair?I don't know the precise scope of the understanding that this couple had when they got married. It wasn't a vow to gladly accept your partner's outside relationships, was it? It was the absence — or rejection — of a vow. But why? Perhaps it was some hippie-style amorphous philosophical belief that one person can't really own the other or that no one at a given time can honestly say for sure where they will be for the rest of their lives.
Maybe, if he never agreed to that particular rule, there's some argument against saying that Mark Sanford cheated. But what can it mean to say that it's "unfair" for her to feel "unhappy"? You feel what you feel, and your actions based on that feeling might be unfair, but if he chose a marriage that did not bind him, why was she bound to anything? She had her feelings, and like him, she gets to go where her emotions lead.