Leo 13th. Smart man.

From the beginning of the world, indeed, it was divinely ordained that things instituted by God and by nature should be proved by us to be the more profitable and salutary the more they remain unchanged in their full integrity. For God, the Maker of all things, well knowing what was good for the institution and preservation of each of His creatures, so ordered them by His will and mind that each might adequately attain the end for which it was made. If the rashness or the wickedness of human agency venture to change or disturb that order of things which has been constituted with fullest foresight, then the designs of infinite wisdom and usefulness begin either to be hurtful or cease to be profitable, partly because through the change undergone they have lost their power of benefiting, and partly because God chooses to inflict punishment on the pride and audacity of man.

Now, those who deny that marriage is holy, and who relegate it, stripped of all holiness, among the class of common secular things, uproot thereby the foundations of nature, not only resisting the designs of Providence, but, so far as they can, destroying the order that God has ordained. No one, therefore, should wonder if from such insane and impious attempts there spring up a crop of evils pernicious in the highest degree both to the salvation of souls and to the safety of the commonwealth.

- Arcanum ("Of the Hidden Things") (on Marriage)
  Papal Encyclical Issued by His Holiness Leo XIII, p.p., Vicarius Christi, 1880