20 Wherefore, conjugal faith, or honor, demands in the first place the complete unity of matrimony which the Creator Himself laid down in the beginning when He wished it to be not otherwise than between one man and one woman. And although afterwards this primeval law was relaxed to some extent by God, the Supreme Legislator, there is no doubt that the law of the Gospel fully restored that original and perfect unity, and abrogated all dispensations as the words of Christ and the constant teaching and action of the Church show plainly. With reason, therefore, does the Sacred Council of Trent solemnly declare: "Christ Our Lord very clearly taught that in this bond two persons only are to be united and joined together when He said: 'Therefore they are no longer two, but one flesh'."
Source: Casti Connubii (Vatican.va)