Friedrich Nietzsche wrote on promise-keeping in Genealogy of Morals as being a result of a refined, and thus cultivated “breeding” of our species, rather than as an innate part of human nature that can be taken out for a spin on day one. In contrast, lying is more expedient and thus primitive in our nature, as if an instinctual urge that is more reflex than refined. In terms of romantic relationships, whether marital or not, “cheating” sexually is definitely a sign of weakness because it places momentary pleasure above being held as reliable (i.e., trustworthy) in terms of promise-keeping. A boyfriend who admits that he might hurt his girlfriend emotionally by engaging in infidelity is really telling her that he is weak-willed and thus not good boyfriend (or husband) material. To be sure, a couple could agree that both can have sex separately with other people, so not being monogamous need not involve violating a promise. I don’t think Nietzsche’s philosophy goes so far as to embrace such an arrangement (especially if romantic feeling or connection is allowed in the separate sex), but neither is monogamy advocated, given the nature of Nietzschean strength that should be allowed out of the cage of societal convention periodically (but not on a regular basis). The concept of strength plays such a powerful role in Nietzsche’s philosophy that even the occasional raw expressiveness of strength beyond a societal straitjacket of moral convention would not be viewed as violating promised monogamy.
The full essay is at "Nietzsche on Promise-Keeping."
