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’.”

Nor did Christ Our Lord wish only to condemn any form of polygamy or polyandry, as they are called, whether successive or simultaneous, and every other external dishonorable act, but, in order that the sacred bonds of marriage may be guarded absolutely inviolate, He forbade also even willful thoughts and desires of such like things: “But I say to you, that whosoever shall look on a woman to lust after her hath already committed adultery with her in his heart.” Which words of Christ Our Lord cannot be annulled even by the consent of one of the partners of marriage for they express a law of God and of nature which no will of man can break or bend.

Nay, that mutual familiar intercourse between the spouses themselves, if the blessing of conjugal faith is to shine with becoming splendor, must be distinguished by chastity so that husband and wife bear themselves in all things with the law of God and of nature, and endeavor always to follow the will of their most wise and holy Creator with the greatest reverence toward the work of God. – from Casti Connubii (1930)