24 Marriage, moreover, is a sacrament, because it is a holy sign which gives grace, showing forth an image of the mystical nuptials of Christ with the Church. But the form and image of these nuptials is shown precisely by the very bond of that most close union in which man and woman are bound together in one; which bond is nothing else but the marriage itself. Hence it is clear that among Christians every true marriage is, in itself and by itself, a sacrament; and that nothing can be further from the truth than to say that the sacrament is a certain added ornament, or outward endowment, which can be separated and torn away from the contract at the caprice of man. Neither, therefore, by reasoning can it be shown, nor by any testimony of history be proved, that power over the marriages of Christians has ever lawfully been handed over to the rulers of the State. If, in this matter, the right of anyone else has ever been violated, no one can truly say that it has been violated by the Church. Would that the teaching of the naturalists, besides being full of falsehood and injustice, were not also the fertile source of much detriment and calamity! But it is easy to see at a glance the greatness of the evil which unhallowed marriages have brought, and ever will bring, on the whole of human society.
Source: Arcanum Divinae (Vatican.va)