On one hand, the Sages greatly emphasize the gravity of this sin. They go so far as to say, “Whoever wastefully ejaculates seed is liable to be put to death” (Nidda 13a). They also state that someone who wastes his seed is like a spiller of blood and an idol-worshiper (ibid). He is like a murderer because he is wasting seed that is meant to add life to the world. He is like an idol-worshiper because instead of increasing love, he is misdirecting the drive for life to self-centered lust, just as idol-worshipers misdirect the power of faith to the worship of wood and stone (see Maharal, Ḥidushei Aggadot, Nidda 13b.)
Zohar goes even further, saying that one who violates this prohibition will not experience the Divine Presence or get eternal reward, since he is like a person who murdered his children. He has spilled a great deal of blood, since this sin becomes habitual. One can repent for all other sins in the Torah, except this one. Nevertheless, if one makes great efforts to repent, motivated by love of God, he will succeed in atoning even for this sin (Zohar I 219b, 62a). Based on Zohar, Shulḥan Arukh states, “This sin is worse than all the other sins in the Torah” (EH 23:1).
On the other hand, there is no explicit Torah prohibition against wasting seed. The verse simply mentions that Er and Onan were punished by death for committing this sin. As a result, the poskim debate whether it is a rabbinic or Torah prohibition. Even according to those who maintain that the prohibition is from the Torah, it is clearly not considered one of the most severe ones, since it is punished neither by death nor by lashes (n. 1 above).
Practical halakhic rulings reflect this as well. If a man sees that his sexual desire is overpowering him to the extent that he will give in to the temptation of adultery or have sexual relations with his wife when she is nidda, it is preferable that he masturbate and then fast to atone for the sin, rather than succumb to these graver sins (Sefer Ḥasidim §176; SA ḤM 23:1; Beit Shmuel ad loc. 1). It is also preferable to masturbate than give in to the temptation of having relations with an unmarried woman, even if she is ritually pure (Responsa Maharshag §243).
Whoever wastes seed is considered wicked and will not experience the Shekhina, as its says, “For You are not a God Who desires wickedness; evil cannot abide with You” (Tehilim 5:5)…. Woe unto that wicked person, for he is evil and he made himself evil. As his hands have dealt, so shall it be done to him (see Yeshayahu 3:11) – This includes someone who commits adultery with his hands (i.e., masturbates), ejaculating and destroying his seed for no reason. He will be punished more harshly in the supernal world than all the other wicked people…. All other wicked souls ascend from hell after they are purged, but he will not…because he literally killed his children and spilled much blood (as one who commits this sin usually does so innumerable times)…. R. Yehuda said, “There is no sin in the world without the possibility of repentance, except for this one, and there are no wicked people who will not experience the Shekhina, except this one, for ‘evil cannot abide with You’ is applied to him.” (Zohar on Parshat Vayeḥi, I, 219b)
However, powerful repentance out of love is effective even for this sin, as it is further stated:
Come and see: For all of a person’s sins and all the damage he caused above, fixing it is dependent upon repentance. But when one commits the sin of spilling seed on the ground and corrupting his ways, he corrupts himself and corrupts the land. About him it is said, “The stain of your sin is before Me” (Yirmiyahu 2:22). It also says, “For You are not a God Who desires wickedness; evil cannot abide with You” (Tehilim 5:5) – unless he repents powerfully. (Zohar on Parshat No’aḥ, I, 62a)
Earlier, Zohar on Parshat Bereishit (I, 56b) states: “They said of the generation of the flood that they committed every sin, but their fate was not sealed until they spilled their seed on the ground for no reason.” On Parshat Vayeshev, Zohar (I, 188a) states:
Come and see: Of all the sins which make a person impure in this world, the sin that defiles man the most, in this world and the next, is spilling seed into the open, ejaculating his seed for no reason using his hands or feet. This makes him impure, as it says, “For You are not a God Who desires wickedness.” For this reason, he cannot enter past the partition and cannot see the face of the Ancient One…. Fortunate is the lot of the man who fears God and avoids the evil path. He purifies himself, doing his best to revere God…and makes an effort to have children with a worthy wife…so he can teach them to walk in the ways of God.
To put this in perspective, it is important to add that just as a waster of seed is said to be like a spiller of blood, so is one who does not fulfill the mitzva of procreation (Yevamot 63b); one who does not visit the sick (Nedarim 40a); who does not accompany a guest upon his departure (Sota 46b); or who embarrasses another person in public (Bava Metzi’a 58b). A community that delays giving charity on account of a fast day is also in this category (Sanhedrin 35a). Additionally, just as someone who wastes seed is compared to an idolater, so is someone who turns a blind eye to the poor and needy (Ketubot 68a and Bava Batra 10a); someone who lives outside Eretz Yisrael (Ketubot 110b); and someone who is arrogant (Sota 4b). All these sins betray the same profound character flaw that can lead to the commission of these grave acts – murder and idol worship – even though in practice the other sins are not as grave, and one need not give up his life to avoid committing these sins, unlike bloodshed and idolatry.
.According to Eliya Rabba 3:15, wasting seed is the most serious of sins when a person causes it by stimulating himself. However, if he feels desire for a married woman or a nidda, it is better for him to masturbate rather than commit the graver sin. Rav Kook explains that wasting seed is more serious than all other sins only when the seed is actually wasted. However, if it serves a purpose, such as preventing the person from having relations with a married woman, the sin is less grave (Ezrat Kohen §35). Still, this seems difficult: how can it be that, on one hand, this sin is described as the gravest of all sins, yet, on the other hand, this prohibition is, at worst, a negative commandment for which the Torah prescribed no punishment? Rather, it must be that the severity of this sin is in the fact that it distances a person from God and prevents him from advancing spiritually. Nevertheless, concerning the punishment in this world and the next, it is less severe than the punishments for which the Torah prescribes death or lashes. R. Tzadok discusses this at length in Takanat Ha-shavin (15:11-12; 15:38). One of the things he explains is Zohar’s statement that someone who commits this sin does not ascend to heaven. I will explain this in the next section.