321. The sixth commandment is: "Thou shalt not commit adultery", and the ninth: "Thou shalt not covet thy neighbor's wife". These two commandments regard the proper propagation of life; hence their great importance. God said to our first parents, "Increase and multiply, and fill the earth" (Gen. II, 28). For that purpose He united them by the bond of matrimony (n. 274). Now by the sixth commandment He forbids to the unmarried every kind of lustful action, as unnatural in their condition; and to the married He forbids any perversion of the marital relations, as gross violations of their sacred bond. The ninth commandment forbids all wilful desires and sensuous thoughts of whatever is forbidden by the sixth. Immodest words, looks, and actions are sinful, in as far as they are likely to cause assent of the will to unchaste pleasure. Of such looks Christ says: "Whosoever shall look on a woman to lust after her, has already committed adultery with her in his heart" (Matt. V, 28). Of immodest language St. Paul writes: "Fornication, and all uncleanness, or covetousness, let it not so much as be named among you, as becometh saints" (Eph. V, 31).
322. The sin by which any lustful pleasure is deliberately admitted is always grievous, and admits no smallness of matter: even a little virus of small-pox or diphtheria is enough to destroy the most vigorous life of the body; thus also each one of these sins, if fully wilful, not only can, but does kill the soul. Often a new species of sin is added to impurity; for when this sin is committed with relatives, it becomes incest; when with married persons, adultery; with those of the same sex, sodomy; etc. When the persons concerned in the sin are consecrated to God, or the sin is committed in a sacred place, it is a sacrilege.
How odious impurity is to God is apparent from the punishment which He has inflicted on those guilty of it -- in particular from the history of the Deluge (Gen. VI), of the destruction of Sodom and Gomorrha (ib. XIX), -- and from the words of St. Paul, who says that those guilty of this sin shall not possess the kingdom of God (1 Cor. VI, 9, 10). Another of its deplorable evils is its tendency to become a tyrannous habit, gaining strength with every indulgence of its cravings, and thus becoming a source of numberless sins. It often ruins honor, fortune, health, mind, and not seldom brings on loathsome diseases and a premature death. It causes scandals, quarrels, bloodshed, incredulity, hardness of heart, and final impenitence. St. Liguori thinks that the greatest number of the lost owe their condemnation to this vice.
323. The chief safeguards against impurity are: 1. Careful avoidance of all unnecessary occasions of immodest thoughts; such are frivolous reading, witnessing immodest shows, indulging in indelicate amusements, dangerous conversations, imprudent familiarities, etc. 2. Daily prayer for grace to resist temptations, especially ejaculatory prayers when danger is nigh. 3. Fervent devotion to the Blessed Virgin, St. Joseph, our Guardian Angel, St. Aloysius, etc. 4. The frequent reception of the Sacraments of Penance and Holy Communion. 5. Cultivating the habit of guarding the eyes, thus imitating the example of Holy Job, who says of himself: "I made a covenant with my eyes, that I would not so much as think of a virgin" (XXXI, 1). 6. Reading the lives of the Saints. 7. Associating with those only whose heart is clean, as can easily be known from their habitual conversation; for "out of the abundance of the heart the mouth speaketh" (Luke VI, 45).