From The Glories of Mary
by St. Alphonsus De Liguori
"Continue, O fool, says St. Peter Damian (speaking to the unchaste), continue to gratify the flesh; for the day will come in which thy impurities will become as pitch in thy entrails, to increase and aggravate the torments of the flame which will burn thee in Hell. The day will come, yea rather the night, when thy lust shall be turned into pitch, to feed in thy bowels the everlasting fire."

Condemnation of the Sin of Sodomy
from Liber Gomorrhianus
by St. Peter Damian
The vice of sodomy "surpasses the enormity of all others," because: "Without fail, it brings death to the body and destruction to the soul. It pollutes the flesh, extinguishes the light of the mind, expels the Holy Spirit from the temple of the human heart, and gives entrance to the devil, the stimulator of lust: It leads to error, totally removes truth from the deluded mind . . . It opens up Hell and closes the gates of Paradise . . . It is this vice that violates temperance, slays modesty, strangles chastity, and slaughters virginity . . . It defiles all things, sullies all things, pollutes all things . . .
"This vice excludes a man from the assembled choir of the Church . . . it separates the soul from God to associate it with demons: This utterly diseased queen of Sodom renders him who obeys the laws of her tyranny infamous to men and odious to God . . . She strips her knights of the armor of virtue, exposing them to be pierced by the spears of every vice. . .
She humiliates her slave in the church and condemns him in court; she defiles him in secret and dishonors him in public; she gnaws at his conscience like a worm and consumes his flesh like fire . . . this unfortunate man [he] is deprived of all moral sense, his memory fails, and the mind's vision is darkened.
Unmindful of God, he also forgets his own identity. This disease erodes the foundation of faith, saps the vitality of hope, dissolves the bond of love. It makes way with justice, demolishes fortitude, removes temperance, and blunts the edge of prudence.

Regarding this vice among clerics
"For God's sake, why do you damnable sodomites pursue the heights of ecclesiastical dignity with such fiery ambition?" . . . . "lest by your prayers you more sharply provoke Him Whom your wicked life so obviously offends."

The sixth Commandment:
"You shall not commit adultery."

"You have heard that it was said, 'You shall not commit adultery.'
But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart." (Mt. 5:27-28)
“You shall not commit impure acts”
492. What are the principal sins against chastity?
2351-2359
2396
Grave sins against chastity differ according to their object: adultery, masturbation, fornication, pornography, prostitution, rape, and homosexual acts. These sins are expressions of the vice of lust. These kinds of acts committed against the physical and moral integrity of minors become even more grave. Compendium of the Catechism.
493. Although it says only “you shall not commit adultery” why does the sixth commandment forbid all sins against chastity?
2336
Although the biblical text of the Decalogue reads “you shall not commit adultery” (Exodus 20:14), the Tradition of the Church comprehensively follows the moral teachings of the Old and New Testaments and considers the sixth commandment as encompassing all sins against chastity.

What the Catholic Church Teaches
"Lust is disordered desire for or inordinate enjoyment of sexual pleasure. Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes.
By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. Both the Magisterium of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action.
The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose. For here sexual pleasure is sought outside of the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved.
Fornication is carnal union between an unmarried man and an unmarried woman. It is gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses and the generation and education of children.
Moreover, it is a grave scandal when there is corruption of the young.
Pornography consists in removing real or simulated sexual acts from the intimacy of the partners, in order to display them deliberately to third parties.
It offends against chastity because it perverts the conjugal act, the intimate giving of spouses to each other. It does grave injury to the dignity of its participants, (actors, vendors, the public), since each one becomes an object of base pleasure and illicit profit for others.
It immerses all who are involved in the illusion of a fantasy world.
It is a grave offense.
Civil authorities should prevent the production and distribution of pornographic materials."
2355 Prostitution does injury to the dignity of the person who engages in it, reducing the person to an instrument of sexual pleasure. The one who pays sins gravely against himself: he violates the chastity to which his Baptism pledged him and defiles his body, the temple of the Holy Spirit.140 Prostitution is a social scourge. It usually involves women, but also men, children, and adolescents (The latter two cases involve the added sin of scandal.). While it is always gravely sinful to engage in prostitution, the imputability of the offense can be attenuated by destitution, blackmail, or social pressure.
2356 Rape is the forcible violation of the sexual intimacy of another person. It does injury to justice and charity. Rape deeply wounds the respect, freedom, and physical and moral integrity to which every person has a right. It causes grave damage that can mark the victim for life. It is always an intrinsically evil act. Graver still is the rape of children committed by parents (incest) or those responsible for the education of the children entrusted to them.
- Catechism of the Catholic Church (2351-56) 224
What does the ninth Commandment forbid?
The ninth Commandment forbids all wilful consent to impure thoughts and desires, and all wilful pleasure in the irregular motions of the flesh
What sins commonly lead to the breaking of the sixth and ninth Commandments?
The sins that commonly lead to the breaking of the sixth and ninth Commandments are gluttony, drunkenness, and intemperance, and also idleness, bad company, and the neglect of prayer.