Being in an exclusive relationship does not make you automatically form feelings for someone. Likewise, being sexual with more than one person does not limit the love, loyalty, or admiration a person has for his or her partner. Monogamy has nothing to do with the feelings or devotion you have for your partner because these feelings do not arise from solely "belonging" to them but, instead, from the experiences that have shaped you both into a different, better, person.
I don't believe monogamy is the only way to be devoted to someone. There are different ways to show your love and to show your loyalty. You can be loyal by standing up for, respecting, and by wanting the absolute best for your partner. But to most, it is not uncommon to associate polygamy with "infidelity". Infidelity is the violation of trust between two intimate persons by being unfaithful. But solely, the act of being polygamous does not label you unfaithful. Therefore, infidelity is defined by the rules and limits that have been agreed upon in a relationship.
Monogamy is not a requirement for love nor is it a requirement for a happy relationship as long as the persons involved have established their expectations and boundaries for each other.