On the nature and dignity of Christian marriage.
By matrimony, therefore, the souls of the contracting parties are joined and knit together more directly and more intimately than are their bodies, and that not by any passing affection of sense of spirit, but by a deliberate and firm act of the will; and from this union of souls by God's decree, a sacred and inviolable bond arises. Hence the nature of this contract, which is proper and peculiar to it alone, makes it entirely different both from the union of animals entered into by the blind instinct of nature alone in which neither reason nor free will plays a part, and also from the haphazard unions of men, which are far removed from all true and honorable unions of will and enjoy none of the rights of family life.
From this it is clear that legitimately constituted authority has the right and therefore the duty to restrict, to prevent, and to punish those base unions which are opposed to reason and to nature; but since it is a matter which flows from human nature itself, no less certain is the teaching of Our predecessor, Leo XIII of happy memory: "In choosing a state of life there is no doubt but that it is in the power and discretion of each one to prefer one or the other: either to embrace the counsel of virginity given by Jesus Christ, or to bind himself in the bonds of matrimony. To take away from man the natural and primeval right of marriage, to circumscribe in any way the principal ends of marriage laid down in the beginning by God Himself in the words 'Increase and multiply,' is beyond the power of any human law."
Therefore the sacred partnership of true marriage is constituted both by the will of God and the will of man. From God comes the very institution of marriage, the ends for which it was instituted, the laws that govern it, the blessings that flow from it; while man, through generous surrender of his own person made to another for the whole span of life, becomes, with the help and cooperation of God, the author of each particular marriage, with the duties and blessings annexed thereto from divine institution. - Casti Connubii 7-9, Pius XI