73 It follows therefore that they are destroying mutual fidelity, who think that the ideas and morality of our present time concerning a certain harmful and false friendship with a third party can be countenanced, and who teach that a greater freedom of feeling and action in such external relations should be allowed to man and wife, particularly as many (so they consider) are possessed of an inborn sexual tendency which cannot be satisfied within the narrow limits of monogamous marriage. That rigid attitude which condemns all sensual affections and actions with a third party they imagine to be a narrowing of mind and heart, something obsolete, or an abject form of jealousy, and as a result they look upon whatever penal laws are passed by the State for the preserving of conjugal faith as void or to be abolished. Such unworthy and idle opinions are condemned by that noble instinct which is found in every chaste husband and wife, and even by the light of the testimony of nature alone, - a testimony that is sanctioned and confirmed by the command of God: "Thou shalt not commit adultry," and the words of Christ: "Whosoever shall look on a woman to lust after her hath already committed adultery with her in his heart." The force of this divine precept can never be weakened by any merely human custom, bad example or pretext of human progress, for just as it is the one and the same "Jesus Christ, yesterday and today and the same for ever," so it is the one and the same doctrine of Christ that abides and of which no one jot or tittle shall pass away till all is fulfilled.
Source: Casti Connubii (Vatican.va)