I'm a big fan of sexual monogamy, but I'm puzzled by lovers who claim that their romantic partner is the only person they need in their lives or that time together is the only activity necessary for emotional fulfillment. Humans are designed to live in groups, explore ideas, and constantly learn new skills. Trying to get all this input from one person is like trying to get a full range of vitamins by eating only ice cream. When a couple believes "We must fulfill all of each other's needs, each becomes exhausted by the effort to be all things to the other and neither can develop fully as an individual.
Sacrificing all our individual needs doesn't strengthen a relationship. Mutually supporting each other's personal growth does.