"Believe"? Odd choice of words. I see monogamy as a characteristic of some relationships, not as a necessary ideal to strive for or put belief into.
Monogamy either is or isn't part of a relationship between two people. Whether a couple can maintain monogamy, I'd say it depends on the persons involved, what they want, and their circumstances. In two-party relationship, it may be important for the two parties to define the extent of their emotional and sexual openness / closedness.
For myself, I'm not inclined toward a two-party romantic relationship. The closest I get to it is that I'm interested in friendships that include sexual communication; it'd be ludicrous for me to expect such friends to be monogamous toward me given that I wouldn't be monogamous.