80 Even by the light of reason alone and particularly if the ancient records of history are investigated, if the unwavering popular conscience is interrogated and the manners and institutions of all races examined, it is sufficiently obvious that there is a certain sacredness and religious character attaching even to the purely natural union of man and woman, "not something added by chance but innate, not imposed by men but involved in the nature of things," since it has "God for its author and has been even from the beginning a foreshadowing of the Incarnation of the Word of God." This sacredness of marriage which is intimately connected with religion and all that is holy, arises from the divine origin we have just mentioned, from its purpose which is the begetting and education of children for God, and the binding of man and wife to God through Christian love and mutual support; and finally it arises from the very nature of wedlock, whose institution is to be sought for in the farseeing Providence of God, whereby it is the means of transmitting life, thus making the parents the ministers, as it were, of the Divine Omnipotence. To this must be added that new element of dignity which comes from the sacrament, by which the Christian marriage is so ennobled and raised to such a level, that it appeared to the Apostle as a great sacrament, honorable in every way.
Source: Casti Connubii (Vatican.va)