Well, of course one reason that masturbation is gravely sinful is that it frustrates the end of sexual intercourse–which seeks procreation. However, in the Church’s teaching on human sexuality, there is more importance than simply the preservation of the end (procreation) because in some cases, procreation is subjectively impossible. And then what comes of relations?
Marital relations are the most perfect expression of *eros *we know, by which Christ allows us to come to know and share His divine love,
agape. In the cleaving of a husband to his wife and a wife to her husband and the freely giving of oneself understood by that, the two become one flesh and share in a level of unity unattained by any other human expression.
Masturbation, being selfish and solitary as it is, frustrates not only the end but the means, which are vastly significant unto themselves.
As for reading material, I would suggest that you read, above all, Pope John Paul the Great’s teachings as they are presented in
Theology of the Body. Get a copy of Christopher West’s
Introduction to the Theology of the Body to read alongside it. Until that arrives, you should read Pope Benedict XVI’s encyclical *
Deus Caritas Est *which contains, in my opinion, one of the most beautiful explanations on God’s love as expressed through marital relatons I have ever read.
Those materials primarily address human sexuality and sexual relations. Understanding the sinfulness of masturbation and other offenses to chastity all rely on a primary understanding of the meaning of human sexuality as God created it, coming back to the marriage bed and the fundamentals of the unity between a husband and wife. From there, we can proceed to understand why masturbation, homosexuality, and fornication, to name a few, are so offensive.
God bless!