6 Yet although matrimony is of its very nature of divine institution, the human will, too, enters into it and performs a most noble part. For each individual marriage, inasmuch as it is a conjugal union of a particular man and woman, arises only from the free consent of each of the spouses; and this free act of the will, by which each party hands over and accepts those rights proper to the state of marriage, is so necessary to constitute true marriage that it cannot be supplied by any human power. This freedom, however, regards only the question whether the contracting parties really wish to enter upon matrimony or to marry this particular person; but the nature of matrimony is entirely independent of the free will of man, so that if one has once contracted matrimony he is thereby subject to its divinely made laws and its essential properties. For the Angelic Doctor, writing on conjugal honor and on the offspring which is the fruit of marriage, says: "These things are so contained in matrimony by the marriage pact itself that, if anything to the contrary were expressed in the consent which makes the marriage, it would not be a true marriage."
Source: Casti Connubii (Vatican.va)