36 If we wish with all reverence to inquire into the intimate reason of this divine decree, Venerable Brethren, we shall easily see it in the mystical signification of Christian marriage which is fully and perfectly verified in consummated marriage between Christians. For, as the Apostle says in his Epistle to the Ephesians, the marriage of Christians recalls that most perfect union which exists between Christ and the Church: "Sacramentum hoc magnum est, ego autem dico, in Christo et in ecclesia;" which union, as long as Christ shall live and the Church through Him, can never be dissolved by any separation. And this St. Augustine clearly declares in these words: "This is safeguarded in Christ and the Church, which, living with Christ who lives for ever may never be divorced from Him. The observance of this sacrament is such in the City of God . . . that is, in the Church of Christ, that when for the sake of begetting children, women marry or are taken to wife, it is wrong to leave a wife that is sterile in order to take another by whom children may be hand. Anyone doing this is guilty of adultery, just as if he married another, guilty not by the law of the day, according to which when one's partner is put away another may be taken, which the Lord allowed in the law of Moses because of the hardness of the hearts of the people of Israel; but by the law of the Gospel."
Source: Casti Connubii (Vatican.va)