The Catechism in Questions & Answers

739. What is marriage?

Marriage is the lifelong union between a man and woman, as desired by God, and upon which His blessing rests. It also forms the foundation for the family. It is based on a public promise of fidelity that is freely given by both partners. Mutual love and fidelity are indispensable factors in the success of a marriage.
Polygamy (marriage with multiple spouses) is not consistent with Christian doctrine and tradition.