Catechism of the Catholic Church
1644 The love of the spouses requires, of its very nature, the unity and indissolubility of the spouses' community of persons, which embraces their entire life: "so they are no longer two, but one flesh." 153 They "are called to grow continually in their communion through day-to-day fidelity to their marriage promise of total mutual self-giving." 154 This human communion is confirmed, purified, and completed by communion in Jesus Christ, given through the sacrament of Matrimony. It is deepened by lives of the common faith and by the Eucharist received together.
1645 "The unity of marriage, distinctly recognized by our Lord, is made clear in the equal personal dignity which must be accorded to man and wife in mutual and unreserved affection." 155 Polygamy is contrary to conjugal love which is undivided and exclusive. 156
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.