Masturbation

The Catholic Church has always taught that masturbation is an intrinsically and gravely disordered action and that the deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose.

Catechism Of The Catholic Church

2351 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.

2352 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."
To form an equitable judgment about the subjects' moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors that lessen or even extenuate moral culpability.

What Did Jesus Teach?

“You have heard that it was said, ‘You shall not commit adultery.’ But I say to you, everyone who looks at a woman with lust has already committed adultery with her in his heart. If your right eye causes you to sin, tear it out and throw it away. It is better for you to lose one of your members than to have your whole body thrown into Gehenna. And if your right hand causes you to sin, cut it off and throw it away. It is better for you to lose one of your members than to have your whole body go into Gehenna. (Matthew 5:27-30)

What Does The Rest Of The Bible Teach?

Judah got a wife named Tamar for his firstborn, Er. But Er, Judah’s firstborn, greatly offended the LORD; so the LORD took his life. Then Judah said to Onan, “Have intercourse with your brother’s wife, in fulfillment of your duty as brother-in-law (Deut. 25:5-10), and thus preserve your brother’s line.” Onan, however, knew that the offspring would not be his; so whenever he had intercourse with his brother’s wife, he wasted his seed on the ground, to avoid giving offspring to his brother. What he did greatly offended the LORD, and the LORD took his life too. (Genesis 38:6-10)

What Did The Church Fathers Teach?
(The following quotations can be found at Churchfathers.org)

“Moreover, he [Moses] has rightly detested the weasel [Lev. 11:29]. For he means, ‘Thou shall not be like to those whom we hear of as committing wickedness with the mouth with the body through uncleanness [orally consummated sex]; nor shall thou be joined to those impure women who commit iniquity with the mouth with the body through uncleanness.’” (Letter of Barnabas10:8 [A.D. 74])  

"Because of its divine institution for the propagation of man, the seed is not to be vainly ejaculated, nor is it to be damaged, nor is it to be wasted." (Clement of Alexandria, The Instructor of Children 2:10:91:2 [A.D. 191])

"To have coitus other than to procreate children is to do injury to nature." (ibid. 2:10:95:3)

"God gave us eyes not to see and desire pleasure, but to see acts to be performed for the needs of life; so too, the genital ['generating'] part of the body, as the name itself teaches, has been received by us for no other purpose than the generation of offspring." (Lactantius, Divine Institutes 6:23:18 [A.D. 307])

"They [certain Egyptian heretics] exercise genital acts, yet prevent the conceiving of children. Not in order to produce offspring, but to satisfy lust, are they eager for corruption." (Epiphanius of Salamis, Medicine Chest Against Heresies 26:5:2 [A.D. 375])

“For thus the eternal law, that is, the will of God creator of all creatures, taking counsel for the conservation of natural order, not to serve lust, but to see to the preservation of the race, permits the delight of mortal flesh to be released from the control of reason in copulation only to propagate progeny.” (Augustine, Against Faustus 22:30 [A.D. 400])

What Have The Popes And The Magisterium Taught?

The traditional Catholic doctrine that masturbation constitutes a grave moral disorder is often called into doubt or expressly denied today. It is said that psychology and sociology show that it is a normal phenomenon of sexual development, especially among the young. It is stated that there is real and serious fault only in the measure that the subject deliberately indulges in solitary pleasure closed in on self ("ipsation"), because in this case the act would indeed be radically opposed to the loving communion between persons of different sex which some hold is what is principally sought in the use of the sexual faculty.
This opinion is contradictory to the teaching and pastoral practice of the Catholic Church. Whatever the force of certain arguments of a biological and philosophical nature, which have sometimes been used by theologians, in fact both the Magisterium of the Church - in the course of a constant tradition - and the moral sense of the faithful have declared without hesitation that masturbation is an intrinsically and seriously disordered act. The main reason is that, whatever the motive for acting this way, the deliberate use of the sexual faculty outside normal conjugal relations essentially contradicts the finality of the faculty. For it lacks the sexual relationship called for by the moral order, namely the relationship which realizes "the full sense of mutual self-giving and human procreation in the context of true love." All deliberate exercise of sexuality must be reserved to this regular relationship. Even if it cannot be proved that Scripture condemns this sin by name, the tradition of the Church has rightly understood it to be condemned in the New Testament when the latter speaks of "impurity," "unchasteness" and other vices contrary to chastity and continence.
Sociological surveys are able to show the frequency of this disorder according to the places, populations or circumstances studied. In this way facts are discovered, but facts do not constitute a criterion for judging the moral value of human acts. The frequency of the phenomenon in question is certainly to be linked with man's innate weakness following original sin; but it is also to be linked with the loss of a sense of God, with the corruption of morals engendered by the commercialization of vice, with the unrestrained licentiousness of so many public entertainments and publications, as well as with the neglect of modesty, which is the guardian of chastity.
On the subject of masturbation modern psychology provides much valid and useful information for formulating a more equitable judgment on moral responsibility and for orienting pastoral action. Psychology helps one to see how the immaturity of adolescence (which can sometimes persist after that age), psychological imbalance or habit can influence behavior, diminishing the deliberate character of the act and bringing about a situation whereby subjectively there may not always be serious fault. But in general, the absence of serious responsibility must not be presumed; this would be to misunderstand people's moral capacity.
In the pastoral ministry, in order to form an adequate judgment in concrete cases, the habitual behavior of people will be considered in its totality, not only with regard to the individual's practice of charity and of justice but also with regard to the individual's care in observing the particular precepts of chastity. In particular, one will have to examine whether the individual is using the necessary means, both natural and supernatural, which Christian asceticism from its long experience recommends for overcoming the passions and progressing in virtue.  Persona Humana: Declaration on Certain Questions Concerning Sexual Ethics, Sacred Congregation for the Doctrine of the Faith, Franjo Cardinal Seper - December 29, 1975.