Humans can't control what they feel, you cannot agree to not develop feelings or interests on other people, doing so is inherently dishonest, not to your partner but to yourself. and expecting it from your partner is equally unfair. yes, two people can agree that they expect all their romantic and sexual needs to come from within the couple and promise not to engage romantically or sexually with other people. But if it wasn't because it is the social norm, that would seem a strange and ridiculous expectation.