Masturbation was commonly regarded in the past as a sin of “self-abuse.” But it makes sense to ask why or how a person “abuses him/herself” by masturbating. To answer this question it is most important to realize that our bodies are definitely not tools or instruments that we, human “persons,” use in order to do different things, among them to give us pleasurable experiences. Such a dualistic understanding of human persons and their bodies is widely accepted in secular culture and has influenced many, including some Christians. This understanding sharply differentiates between the “person,” i.e., the subject of experiences, and the “person’s” “body,” which of itself is part of the world of nature over which the “person” has dominion.
But in truth the human body is integral to the being of the human person. Human persons are bodies, and their bodies reveal them as the kind of persons they are. They are not spiritual persons, as are the Three Persons of the Triune God and angels. They are “body” persons and as such are inescapably male or female persons. When Cod created man “he created them male and female,” that is bodily beings, “body persons,” not “spirit persons” like angels or the Persons of the Trinity. They are men or women summoned from their depths to “give” themselves to one another in love.
Because many today regard their own bodies merely as a means of bringing about a desired conscious state and do not respond as bodily persons, as men and women, to the real goods making human sexuality meaningful, masturbators violate the body’s capacity for self-giving, or what John Paul II calls its “nuptial” or “spousal” significance. 
Are there any exceptions?
Some authors, e. g., Anthony Kosnik, et., al. refer to pathological masturbation.  This terminology suggests that masturbation is at times not a freely chosen or voluntary human action but is rather compulsive behavior. We must, however, distinguish between truly compulsive behavior, in which case there simply is no human act, from what can be called “quasi-compulsive” sins of weakness.  With the latter there is indeed a true human act, freely chosen after deliberation, but one of which the agent is ordinarily immediately ashamed, the kind of behavior exhibited, for instance, by alcoholics . 
Other writers¸ e.g., Martin Rhonheimer, think that medically indicated masturbation to procure semen for analysis is permissible because it is morally different from masturbation to procure pleasure.  But this seeks to justify an act because of the end for which it has been deliberately chosen—i.e., it is an instance of “the end justifying the means.” But this objection does not hold water. The inducing of orgasm seems to be of the essence of the act as a moral act — it is by inducing the orgasm that the sperm is procured. Thus, we have, in fact, a disordered sexual act — one directed toward orgasm outside of marital intercourse. The fact that there is a good purpose for such an act does not remove its disorder. Moreover, even if medically indicated masturbation could be shown to be a different sort of act than lustful masturbation —that it is simply an act of obtaining semen for medical purposes and not the performance of a distinctively sexual act — it still does not follow that acts of this type would be permissible. There are, after all, other effective and practical ways to procure sperm.  This activity ordinarily stimulates pleasure, and so inclines the agent to delight in sexual pleasure unrelated to the real goods of sexuality. One cannot masturbate simply by taking thought—one has to do things and to do them efficiently—that is the reason why fertility clinics engaged in the “new reproductive technologies” help the men “donating” sperm for insemination by providing the rooms where they are to masturbate with pornographic materials. “Medically indicated” masturbation is essentially deceptive rhetoric.
This piece challenges the dualistic anthropology that separates the “person” from his or her body and seeks to justify masturbation as the use by the “person,” i.e. consciously experiencing subject, of his/her body as a subpersonal tool or instrument to provide him/her with a pleasurable experience that harms no one. Emphasizing the truth that human persons are “body persons” whose bodies are integral to their being as persons¸it shows that deliberately choosing to masturbate seriously violates the meaning of the body, male and female, as a “gift” to be given in lasting love to a person of the opposite sex in marriage, i.e., it violates the “nuptial” or “spousal” meaning of the body.
 See Pope John Paul II, Man and Woman He Created Them: A Theology of the Body, Translation, Introduction, and Index by Michael Waldstein (Boston: Pauline Books and Media, 2006), pp. 178-190.
 Anthony Kosnik et al. Human Sexuality: New Directions in American Catholic Thought (Paramus, N. J.: Paulist Press, 1977), pp.225 f.
 Germain Grisez, Christian Moral Principles (Staten Island: Alba House, 2005 reprint), Ch. 17, questions D through E, pp. 419-427.
 Martin Rhonheimer, “Intentional Actions and the Meaning of Object: A Reply to Richard McCormick,” in Veritatis Splendor and the Renewal of Moral Theology, eds. J. A. DiNoia, and Romanus Cessario, (Chicago: Midwest Theological Forum, 1999), pp. 266-267.
 H.P. Dunn, Ethics for Doctors, Nurses, and Patients (Staten Island: Alba House, 1994), pp. 103-104
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