7 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.
Source: Casti Connubii (Vatican.va)