The spilling of the seed that denies life to future generations. The perpetrator will be felled by God; he cannot be punished by a human court. Scripture tells us of Onan, who spilled his seed in order not to give children to his brother’s widow; God killed him (Gen. 38:9-10). His name has long been attached to the practice of self-gratification: onanism. It must be remembered that Onan’s act was motivated by ulterior considerations. By denying children to his brother’s widow, he himself stood to inherit a portion of his brother’s estate. Rabbinic injunction, however, is based on the act itself and sees it as forbidden by Torah.
Under contemporary conditions, many of the acts that Torah prohibits no longer have a religious character. Masturbation, for example, has been recognized as harmless to health, and it is not motivated by the concerns that prompted Onan on the occasion reported in Scripture.