On March 21, 1931, the Holy Office (now the Sacred Congregation for the Doctrine of the Faith) issued a decree that contained a response to a question pertaining to Pope Pius XI’s encyclical, Christian Education of Youth.

Question:
May the method called “sex education” or even “sex initiation” be approved?

Answer:
No. In the education of youth the method to be followed is that hitherto observed by the Church and the Saints as recommended by His Holiness the Pope in the encyclical dealing with the Christian education of youth, promulgated on December 31, 1929. The first place is to be given to the full, sound and continuous instruction in religion of both sexes. Esteem, desire and love of the angelic virtue must be instilled into their minds and hearts. They must be made fully alive to the necessity of constant prayer, and assiduous frequenting of the Sacraments of Penance and the Holy Eucharist; they must be directed to foster a filial devotion to the Blessed Virgin as Mother of holy purity, to whose protection they must entirely commit themselves. Precautions must be taken to see that they avoid dangerous reading, indecent shows, conversations of the wicked, and all other occasions of sin.

Hence no approbation whatever can be given to the advocacy of the new method even as taken up recently by some Catholic authors and set before the public in printed publications.

“The traditional norms established in 1929 by Pope Pius XI’s Encyclical Divini Illius Magistri have never been nullified or abrogated, and therefore remain in effect to the present day” (Engel, Sex Education: The Final Plague, 1989, p. 55). This encyclical condemns public, explicit, and naturalistic instruction in human sexuality. All subsequent papal teachings (Magisterium) refer and respect the following landmark document.

Divini Illius Magistri -1929

The 1929 encyclical letter of Pope Pius XI on Christian Education of Youth (Divini Illius Magistri) was the first official condemnation of sex education in this century. Excerpts are listed below:

. . . every form of pedagogic naturalism which in any way excludes or weakens supernatural Christian formation in the teaching of youth, is false. Every method of education if founded, wholly or in part, on the denial or forgetfulness of original sin and of grace, and relying on the sole powers of human nature, is unsound. . .

. . .so today we see, strange sights indeed, educators and philosophers who spend their lives in searching for a universal moral code of education, as if there existed no Decalogue, no Gospel law, no law even of nature stamped by God on the heart of man, promulgated by right reason, and codified in positive Revelation by God Himself in the Ten Commandments. . .

Another very grave danger is that of 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 and ugly term propagate a so-called sex education, falsely imagining they can forearm youth 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 men are miserably deluded in their claim to emancipate, as they say, the child, while in reality, they are making him the slave of his own blind pride and of his own blind disorderly affections.

On the matter of sex instruction

Such persons grievously err in refusing to recognize the inborn weakness of human nature, and the law of which the Apostle speaks, fighting againstthe law of 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 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.
(From the encyclical Christian Education of Youth)