Masturbation/ a sin?

Below is a quote from comment 8 here 

This question is only tangentially related to the topic, but since it is something that many people, men in particular, struggle with and have questions about, I will briefly answer by appealing to Scripture, Tradition, and the Magisterium:

Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God. (Galatians 5:19-21, RSV; emphasis added)

As stated above (Aa. 6,9) wherever there occurs a special kind of deformity whereby the venereal act is rendered unbecoming, there is a determinate species of lust. This may occur in two ways: First, through being contrary to right reason, and this is common to all lustful vices; secondly, because, in addition, it is contrary to the natural order of the venereal act as becoming to the human race: and this is called “the unnatural vice.” This may happen in several ways. First, by procuring pollution, without any copulation, for the sake of venereal pleasure: this pertains to the sin of “uncleanness” which some call “effeminacy.” Secondly, by copulation with a thing of undue species, and this is called “bestiality.” Thirdly, by copulation with an undue sex, male with male, or female with female, as the Apostle states (Rom. 1:27): and this is called the “vice of sodomy.” Fourthly, by not observing the natural manner of copulation, either as to undue means, or as to other monstrous and bestial manners of copulation. (Summa theologica, IIa IIae q. 154 a. 11)

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.” (Catechism of the Catholic Church, 2352)

The “impurity” (or “uncleanness”) mentioned by St. Paul, is a sin somehow distinct from adultery and fornication, and has been taken to be among the “unnatural vices,” under which rubric St. Thomas includes masturbation (“procuring pollution, without any copulation”). The Catechism explicitly uses and further defines the term “masturbation,” and clearly states that the action is “intrinsically and gravely disordered.” Thus, this action, done with deliberate consent and in the knowledge that it is intrinsically and gravely disordered, is a mortal sin.

So the answers to your questions are: Yes it is a sin; no it is not okay; yes, because the action involves grave matter, one must, with contrition and with the purpose to amend one’s life, avail oneself of the grace of sacramental penance; yes, the teaching that masturbation is a sin has always been the teaching of the Church.

There might be a large number of Catholics who do not know that this is the teaching of the Church. Regardless, masturbation involves grave matter, being classified by St. Thomas with the unnatural vices of bestiality, sodomy and contraception. Anyone who wishes to enter the Kingdom of God must flee such forms of immorality.