65. Another very grave danger is that naturalism which nowadays invades the field of education in that most delicate matter of purity of morals. Far too common is the error of those who with dangerous assurance and under an ugly term propagate a so-called sex-education, falsely imagining they can forearm youths against the dangers of sensuality by means purely natural, such as a foolhardy initiation and precautionary instruction for all indiscriminately, even in public; and, worse still, by exposing them at an early age to the occasions, in order to accustom them, so it is argued, and as it were to harden them against such dangers.
66. Such persons grievously err in refusing to recognize the inborn weakness of human nature, and the law of which the Apostle speaks, fighting against the law of the mind; and also in ignoring the experience of facts, from which it is clear that, particularly in young people, evil practices are the effect not so much of ignorance of intellect as of weakness of a will exposed to dangerous occasions, and unsupported by the means of grace.
DIVINI ILLIUS MAGISTRI, Pope Pius XI, 1929
February 10, 2014