(The green part is a quote from elsewhere, included in Elizium23's post.)
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, if not even reduce to a minimum, moral culpability.

The case against masturbation is not solely based on Onan's sin. It is, of course, a strong argument for its Scriptural basis, but the primary principle here is stated in the Catechism. "The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose." This is a consistent and logical application of Divine Law to human sexuality. From this principle we also condemn sodomy, homosexuality, adultery, fornication, pedophilia, sexual fetishes, BDSM, and the like.