Much more pernicious are those entirely naturalistic ideas or teachings which invade the field of education in that most delicate matter of purity of morals and chastity. Far and wide and very numerous are they who foolishly and recklessly guard and promote a type of education which is repugnantly called sexual, wherein they think they can inoculate young people from pleasure and sensuality using merely natural methods, free of every help of religion and piety, with no distinction as to sex and even in group settings, by initiating them and providing them with hazardous information; and worse still, by exposing them at the earliest age that opportunity offers, in order to accustom them, so they say, and as it were to harden them against such dangers.
Such persons grievously err by refusing to recognize the inborn weakness of human nature and that law in our members warring against the law of our mind, of which Paul the Apostle speaks (Romans vii:23); and by blindly denying what we have learned from daily experience, that young people in particular more often fall into immorality not because of defective knowledge, but rather because of weakness of a will exposed to allurements, and lacking divine helps.
In this extremely delicate matter, if, all things thoroughly considered, it may be necessary that some adolescent be counseled at an appropriate time by those to whom God has committed the duty of rearing children and who have the proper graces, every precaution and appropriate conduct must be employed. Such precautions are well known in traditional Christian education, which Antonianus fittingly describes in these words:
Such sadly is our weakness and propensity to sin, that often measures considered as a remedy for sin, themselves provide an incitement and occasion for sin. Hence it is of the highest importance that a prudent father, if ever he may discuss such delicate matters with his son, should be well on his guard not to descend to details or to refer to the individual ways by which this horrible pestilence has infected such a large part of the world with its poison; lest while attempting to extinguish the flame of desire, he may rather arouse in the tender heart of the child what was hitherto dormant, or he may wholly enkindle it. Generally speaking, when children are being brought up, it suffices to employ those aids which bring purity into their hearts and at the same time keep them from contrary vices.
from the encyclical Divini illius Magistri, ¶ 66-69