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.
Source: Casti Connubii (Vatican.va)