From the book of prophet Pius XI:
[S]ince, in order that the deceits of the enemy may be avoided, it is necessary first of all that they be laid bare; since much is to be gained by denouncing these fallacies for the sake of the unwary, even though We prefer not to name these iniquities "as becometh saints," yet for the welfare of souls We cannot remain altogether silent.
To begin at the very source of these evils, their basic principle lies in this, that matrimony is repeatedly declared to be not instituted by the Author of nature nor raised by Christ the Lord to the dignity of a true sacrament, but invented by man. Some confidently assert that they have found no evidence of the existence of matrimony in nature or in her laws, but regard it merely as the means of producing life and of gratifying in one way or another a vehement impulse; on the other hand, others recognize that certain beginnings or, as it were, seeds of true wedlock are found in the nature of man since, unless men were bound together by some form of permanent tie, the dignity of husband and wife or the natural end of propagating and rearing the offspring would not receive satisfactory provision. At the same time they maintain that in all beyond this germinal idea matrimony, through various concurrent causes, is invented solely by the mind of man, established solely by his will.
How grievously all these err and how shamelessly they leave the ways of honesty is already evident from what we have set forth here regarding the origin and nature of wedlock, its purposes and the good inherent in it. The evil of this teaching is plainly seen from the consequences which its advocates deduce from it, namely, that the laws, institutions and customs by which wedlock is governed, since they take their origin solely from the will of man, are subject entirely to him, hence can and must be founded, changed and abrogated according to human caprice and the shifting circumstances of human affairs; that the generative power which is grounded in nature itself is more sacred and has wider range than matrimony - hence it may be exercised both outside as well as within the confines of wedlock, and though the purpose of matrimony be set aside, as though to suggest that the license of a base fornicating woman should enjoy the same rights as the chaste motherhood of a lawfully wedded wife.
Armed with these principles, some men go so far as to concoct new species of unions, suited, as they say, to the present temper of men and the times, which various new forms of matrimony they presume to label "temporary," "experimental," and "companionate." These offer all the indulgence of matrimony and its rights without, however, the indissoluble bond, and without offspring, unless later the parties alter their cohabitation into a matrimony in the full sense of the law.
Indeed there are some who desire and insist that these practices be legitimatized by the law or, at least, excused by their general acceptance among the people. They do not seem even to suspect that these proposals partake of nothing of the modern "culture" in which they glory so much, but are simply hateful abominations which beyond all question reduce our truly cultured nations to the barbarous standards of savage peoples.