From Pope St. John Paul II’s Apostolic Exhortation Familiaris Consortio (1981):
The first communion is the one which is established and which develops between husband and wife: by virtue of the covenant of married life, the man and woman “are no longer two but one flesh”46 and they are called to grow continually in their communion through day-to-day fidelity to their marriage promise of total mutual self-giving.
This conjugal communion sinks its roots in the natural complementarity that exists between man and woman, and is nurtured through the personal willingness of the spouses to share their entire life-project, what they have and what they are: for this reason such communion is the fruit and the sign of a profoundly human need. But in the Lord Christ God takes up this human need, confirms it, purifies it and elevates it, leading it to perfection through the sacrament of matrimony: the Holy Spirit who is poured out in the sacramental celebration offers Christian couples the gift of a new communion of love that is the living and real image of that unique unity which makes of the Church the indivisible Mystical Body of the Lord Jesus.
The gift of the Spirit is a commandment of life for Christian spouses and at the same time a stimulating impulse so that every day they may progress towards an ever richer union with each other on all levels—of the body, of the character, of the heart, of the intelligence and will, of the soul47—revealing in this way to the Church and to the world the new communion of love, given by the grace of Christ.
Such a communion is radically contradicted by polygamy: this, in fact, directly negates the plan of God which was revealed from the beginning, because it is contrary to the equal personal dignity of men and women who in matrimony give themselves with a love that is total and therefore unique and exclusive. As the Second Vatican Council writes: “Firmly established by the Lord, the unity of marriage will radiate from the equal personal dignity of husband and wife, a dignity acknowledged by mutual and total love.”48
46 Mt 19:6; cf. Gn 2:24.
47 Cf. John Paul II, Address to Married People at Kinshasa (May 3, 1980) 4: Acta Apostolicæ Sedis 72 (1980), 426-427.
48 Pastoral Constitution on the Church in the Modern World Gaudium et spes, 49; cf. John Paul II, Address at Kinshasa 4: loc. cit.
— Familiaris Consortio 19