The Touchy Subject of Masturbation

Tanya Chapter 7
Therefore the sin of wasteful emission of semen is not mentioned in the Torah among the list of forbidden coitions, even though it is more heinous than they, and the individual’s sin is greater with regard to the enormity and abundance of the impurity and of the kelipot; he begets and multiplies them to an exceedingly great extent through wasteful emission of semen, even more so than through forbidden coitions….elsewhere, cites a requirement to fast 84 fasts (sunup to sundown, no food or water) for each episode of masturbation.

Elliot Dorff, Matter of Life and Death: A Jewish Approach to Medical Ethics
Masturbation in and of itself, though, should no longer carry the opprobrium it had for our ancestors, both because the original grounds for opposing it are no longer tenable and because it is at least preferable to non- marital sex as a way of dealing with sexual energies before marriage.