Marriage: nature and divine cannot be altered by man

An Excerpt from Pope Pius XI encyclical on Christian Marriage (Casti Connubii)

44. When we consider the great excellence of chaste wedlock, Venerable Brethren, it appears all the more regrettable that particularly in our day we should witness this divine institution often scorned and on every side degraded.

49. 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.

50. 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.