40 In like manner, all ought to understand clearly that, if there be any union of a man and a woman among the faithful of Christ which is not a sacrament, such union has not the force and nature of a proper marriage; that, although contracted in accordance with the laws of the State, it cannot be more than a rite or custom introduced by the civil law. Further, the civil law can deal with and decide those matters alone which in the civil order spring from marriage, and which cannot possibly exist, as is evident, unless there be a true and lawful cause of them, that is to say, the nuptial bond. It is of the greatest consequence to husband and wife that all these things should be known and well understood by them, in order that they may conform to the laws of the State, if there be no objection on the part of the Church; for the Church wishes the effects of marriage to be guarded in all possible ways, and that no harm may come to the children.
Source: Arcanum Divinae (Vatican.va)