"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.
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.
In this extremely delicate matter, if, all things considered, some private instruction is found necessary and opportune, from those who hold from God the commission to teach and who have the grace of state, every precaution must be taken. Such precautions are well known in traditional Christian education, and are adequately described by Antoniano cited above, when he says: Such is our misery and inclination to sin, that often in the very things considered to be remedies against sin, we find occasions for and inducements to sin itself. Hence it is of the highest importance that a good father, while discussing with his son a matter so delicate, should be well on his guard and not descend to details, nor refer to the various ways in which this infernal hydra destroys with its poison so large a portion of the world; otherwise it may happen that instead of extinguishing this fire, he unwittingly stirs or kindles it in the simple and tender heart of the child. Speaking generally, during the period of childhood it suffices to employ those remedies which produce the double effect of opening the door to the virtue of purity and closing the door upon vice."
-Pius XI, Divini Illius Magistri, 65-67.