It is quite well known that a Jew has a prohibition against destroying his (or her) own seed. Although it is unknown (or at least subject to controversy) exactly what the Scriptural source for this prohibition is, it is widely accepted that such a prohibition exists. It is almost universally agreed upon that this was the fatal sin of Onan. Therefore, such actions are sometimes called an “Onania” (or “Onanism”, i.e. in Latin, Coitus interruptus). Rabbi Mordecai ben Avrohom Yoffe (1530-1612) writes that the entire generation of the flood during Noah’s times was guilty of the sin of destroying seed as the verse tells, “For all flesh has destroyed its ways on the earth.” The prophet Isaiah rebuked the Jews of his time by telling them that HaShem did not create the world for emptiness; He formed it to be inhabited. Destroying seed, therefore, is akin to destroying the world because it is the antithesis to the reason behind creation. The Talmud says that it is surely prohibited for a man to ejaculate his semen upon sticks and stones (i.e. not into a female). However, it is unclear to what extent women are included in this prohibition and whether post-relations contraceptive (e.g. an internal sponge to absorb semen to prevent pregnancy) is forbidden.
Most of the early Bible commentaries to Exodus 20:13 (Rashi, Ibn Ezra, Sforno, Sefer HaChinuch, et al.) explain that the prohibition of lo sinaf (the second prohibition in the referenced verse) outlaws adultery and –according to some— other forbidden sexual relations (znus). However, the Talmud, as referenced by the Torah Temimah (an encyclopedic work written by Rabbi Baruch HaLevi Epstein, 1860-1941), understands that the prohibition outlaws wasteful destruction of seed. In combining both of these opinions, one arrives to the conclusion that there is only an issur (prohibition) of lo sinaf in a situation where there is a znus-type action being performed. If an illegal sexual intercourse is not being carried out (e.g. the case of wood and stone), it serves to be that there is no prohibition of lo sinaf. However, in such a case there is a clear violation of the positive injunction to “be fruitful and multiply”, because that commandment encompasses both a positive aspect of having or creating offspring, and a negative aspect of not doing an action which is contrary to the positive commandment, such as destruction of seed. Nonetheless, since women are not included in this directive of procreation—according to Yevamos 65b which says, based on Genesis 1:28 and the fact that women are not conquerors, that women are exempt for the commandment of populating the world—it seems they can legally violate this law.
Rabbi Yaakov Ettlinger (1798-1871) writes that perhaps the admonition not to destroy one’s sperm stems from the prohibition “Do not destroy its trees,” a general commandment which includes meaningless destruction and self-inflicted harm. Rabbi Reuven Grozovsky (1896-1956) explained that the prohibition of destroying seed is only wasteful destruction. He compared the destruction of human seed to the destruction of fruit bearing trees: Just as the prohibition against destroying fruit treesis only in a wasteful manner with needless destruction, the prohibition of destroying one’s seed is only in a wasteful manner with needless destruction. However, normally, such destructions are usually permitted if and only if they serve a purpose. The Ri, an early Tosafist, is of the opinion that even post-relations contraception constitutes the destruction of seed. However, other feel there can be room to say that such contraception is not considered wasteful destruction of one’s seed because the actual relations were done in a normal manner, which is not comparable to ejaculating upon wood and rocks (which the Talmud cited above explicitly outlawed). The Ri also believes that the prohibition does not apply to women; however, this opinion too is disputed. Rabbeinu Tam (1100-1171) believes that the prohibition of the destruction of seed applies even to women. This is because Rabbenu Tam believed the prohibition is not a part of the commandment of procreation. However, elsewhere he decided that the law is that one who already fulfilled his obligation of procreation has no prohibition to destroy his or her seed. This seems to imply interdependency between the laws of procreation and the prohibition of seed destruction. Therefore, the Ritva (Rabbi Yom Tov ben Avrohom Asevilli, 1250-1330) corrects the text of the passage of Rabbenu Tam in Sefer HaYasher to read the opposite. One can also answer the apparent contradiction within the opinion of Rabbenu Tam by explaining that he understands the prohibition of destruction of seed like Rashi understood it (see below).
One can infer from Rashi that Rashi maintains, like his grandson Rabbeinu Tam, that even a woman has a prohibition to her destroy seed. From the fact that he believes the prohibition extends even to the female population shows that he does not believe the prohibition to be an extension of the commandment of procreation, for women are exempt from that commandment. The Talmud likens the destruction of one’s seed to murder. This seems to imply that the prohibition of destroying seed is the prohibition of murder. Indeed, Rabbi Yechiel Michel Epstein (1829-1907) writes that inside one’s seed is a mini-person and destroying one’s seed is therefore akin to killing that miniature human being. However, in order to say that Rashi believed the prohibition of seed destruction is murder, then one must assume that Rashi believed that there is a prohibition of murder on a fetus and embryo in order to say the prohibition applies even to seed. The existence of the prohibition of murder on a fetus and embryo is quite controversial and Rashi himself seems to be of the opinion that there is no prohibition (at least for a Jew). Therefore, one is forced to conclude that Rashi believed the prohibition of destroying seed is a prohibition of idolatry as the Gemara likens the destruction of one’s seed to idolatry. Indeed the Zohar teaches that one who masturbates is actually offering sacrifices to damaging demons. In any event, according to Rashi, the prohibition of the destruction of seed applies both to the man and to the woman and includes both the man’s seed and the woman’s seed. However, Nachmanides (Rabbi Moshe ben Nachman of Gerondi, 1194-1270) and the Ran (Rabbi Nissim ben Reuven of Girona, 1320-138) believe that only a man’s seed is included in the prohibition, not a woman’s seed. It is physiologically unclear what is meant by the term “woman’s seed” (see below).
Although some modern day scholars believe that the prohibition is Rabbinical in origin, Rabbi Moshe Feinstein (1895-1986) writes that, heaven forbid, one should not think that the prohibition of destroying seed is merely rabbinical in origin. He maintains that it is a biblical prohibition and is even punishable by death, as Rabbi Yochanan said. His source is a passage in the Talmud in which Rabbi Pinchos Ben Yair derived from Deuteronomy 23:10 that one should guard himself from nocturnal emissions. Tosafos understand this interpretation of Rabbi Pinchos Ben Yair to be the proper reading of the verse, thus giving the prohibition biblical status. Additionally, the Mabit (Rabbi Moshe Ben Yosef of Tirani, 1500-1580) feels that the prohibition is biblical. Various quotations from the Zohar also support this assertion. Rabbi Eliezer Yehuda Waldenberg (a prominent judge on the Supreme Rabbinical Court in Jerusalem) quotes in the name of the Meshivas Nefesh (Rabbi Aryeh Leib from Palatzik) that the prohibition is only rabbinic. The Tzitz Eliezer himself feels that Rabbi Yitzchok and Rabbi Ami argue with Rabbi Yochanan’s assertion that destroying seed is punishable by death. Rabbi Yehoshua ben Alexander HaCohen Katz Falk (1555-1614) seems to understand that the prohibition is even less than rabbinic which explains why the Rambam does not even prescribe rabbinic lashes for its violation. However, the prohibition must be, in actuality, a biblical one for if it was only rabbinical, then the decree would have been nonexistent because the Rabbis only proclaim a decree if the majority of the population would be able to withstand it.
It is clear that the “seed” of a man refers to the semen contained within his sperm; however, it is unclear to what exactly the Sages refer to when they discuss the “seed” of a woman. The Rambam (Maimonides) writes that a woman has two testicle-like organs, which produce her “seed.” The Ramban (Nachmanides) argues and writes that although women have “testicles” similar to men, those testicles do not contain any “seed”, or the “seed” that they do contain is never emitted from the woman’s body. The Ben Ish Chai (Chacham Yosef Chaim of Baghdad, 1832-1909) concurred with the first explanation of the Ramban. The Chazon Ish (Avrohom Yeshayahu Karelitz, 1878-1953) writes based on both understandings that the blood emitted by women is referred to as their seed. He says that when women sometimes expel blood because of desire (or fear—see Sotah 20b) this blood is considered their seed. (Perhaps he was referring to the current controversy regarding female ejaculation.) The author of Tosafos Yom Tov (Rabbi Yom-Tov Lipmann ben Nosson ben Moshe ha-Levi Heller, 1579-1654) writes that the “seed” of a woman is an internal blood that is crucial to the child-making process and a certain degree of sexual arousal can rouse this blood. (As a result, he says, lesbians should be liable for violating the prohibition against the destruction of female seed.)
In Scripture’s listing of the descendants of Jacob, the sons are ascribed as sons of their respective mothers, while Jacob’s daughter, Dinah, is described as the daughter of Jacob. The Talmud explains that from here one learns that when the male “sows” first during the intercourse (i.e. emits his seed), sons are born, and, if the female “sows” first during the intercourse (i.e. emits her “seed”), then daughters are born. However, the problem with this understanding is that at the time of her conception, Dina was supposed to be a boy and due to her mother Leah’s prayers, her gender switched. Therefore, the Midrash, Vayikra Rabbah, understands that the source of orgasmic determination of gender is Leviticus 12:2, which implies that if a woman sows, she shall bear a male child. Furthermore, the Talmud writes that there are three partners in childbirth, the first two are the biological parents whose collective “seeds” make up part of their child, and the third partner is HaShem. The Ramban explains that when the Rabbis say that the woman has the ability to sow, which implies the expulsion of “seed”, they mean that the blood within her womb collects itself in order to receive the man’s seed. He relates that the Greek philosophers believed that the entire body of an embryo is made from the blood of its mother, while the man only contributes to certain minor abilities within the child. Rabbeinu Bachya ben Asher (1255-1340) writes that scientifically when a woman’s mind is totally focused on her husband, she is considered to have “sown” first, and, therefore, her conceived child will be a male. This is similar to the method used by Yaakov Aveinu in his attempt to cause Lavan’s goats to be born either speckled or dappled and the sheep to be born brownish.
Rabbi Yaakov Culi (1685-1732) writes that it is debatable whether women are included in the prohibition of destroying seeds or not. He writes that according to Rabbeinu Tam, since women are not included in the comment of procreation, they are not forbidden to destroy human seeds, while according to Nachmanides, despite the fact that women are not commanded to procreate, they are still proscribed against destroying human seed. Amongst the medieval sages, there are two explanation of what exactly is the proscribed act of lesbianism. According to the Rivan, the forbidden act of lesbianism is when a woman emits the seed injected into her by husband in the context of intimate relations with another woman. According to another explanation cited in Tosafos, the act of lesbianism mentioned in the Talmud is the act of Tribadism. Rabbi Yosef Chaim of Baghdad (1835-1909) writes that even according to the first opinion, the prohibition of lesbianism is not because of the seed, which is spilled; rather, lesbianism is prohibited because of a separate prohibition banning all acts that are "Egyptian" in nature. The Mabit, Rabbi Moshe ben Yoseph di Trani the Elder (1505-1585), writes that while the prohibition banning "Egyptian acts" is biblical in origin while the prohibition of spilling one's seed is only possibly biblical, but not necessarily.
Although actions like necrophilia could be technically permitted, the above rationale concerning destroying seed applies to them (and possibly they could also fall under the general prohibition of Leviticus 20:25 which bans all disgusting actions and the commandment of Leviticus 19:2 which orders all Jews to behave holy). Both male and female masturbation is also prohibited based on such reasoning. The Beraisa in the second chapter of Tractate Kallah, of the minor tractates, states that those who destroy their seed delay the coming of the Messiah. This is true because G-d has a treasure chest filled with souls, and only once He finishes pairing each soul with its body will the scion of King David arrive. In the merit of learning these laws and expounding on the Torah, may we live to see the coming of the Messiah speedily and in our days: Amen.
 Genesis 38:9-10, see also Yevamos 34b
 Levush Malchus, Even Ha’ezer §23:1
 Genesis 6:12
 Isaiah 45:18
 Niddah 13b
 Niddah 13a
 Genesis 1:28
 see Minchas Chinuch §1
 A proof to this double feature of the commandment to procreate is Tosafos on Sanhedrin 59b who say that procreation is both an active positive commandment and a passive negative commandment.
 Aruch Laner to Niddah 13b and responsa Binyan Tzion §135
 Deuteronomy 20:19
 Bava Kamma 91b
 Chidduschei Rabbi Reuven to Yevamos 12b
 Deuteronomy 20:19
 Kesubos 39a
 Yevamos 12b
 Sefer HaYashar
 See Gilyon, marginal footnotes to Niddah 13a
 To Niddah 13a
 Kesubos 39a, Yevamos 12b
 Niddah 13b
 Aruch HaShulchan, Even Ha’ezer §23:1
 See Sanhedrin 67b.
 Niddah 13b
 Chiddushei HaRamban to Kesubos 39a
 Chiddushei HaRan, Ibid.
 responsa Iggress Moshe, Even Ha’Ezer, volume 3 §14
 Niddah 13a
 Kesubos 45a
 Avodah Zarah 20b
 See the Mabit’s Kiryas Sefer to Laws of Sexual Prohibitions 21:18
 Responsa Tzitz Eliezer, volume 9 §51
 Meshivas Nefesh §18
 See Niddah 13a
 responsa Pnei Yehoshua, Even HaEzer, volume 2 §44
 Maimonides, Laws of Sexual Prohibition, 21:18
 As is evident throughout the Talmud, see Horayos 3b, Avodah Zarah 36a, and Bava Basra 60b.
 Maimonides, Laws of Sexual Prohibitions 5:4
 Commentary to Leviticus 12:2
 See Ben Yehoyada to Sotah 22b
 See Chazon Ish Yoreh Deah §104:3 and Chazon Ish Even HaEzer §36:3
 Ma’adnei Yom Tov on the Rosh to the third chapter of Niddah §2
 Genesis 46:15
 Niddah 31a
 See Targum Yonason Ben Uziel to Genesis 30:26
 Niddah 31a
 Cited above
 Commentary to Leviticus 12:2
 See Rashi to Genesis 30:37-39. Jacob had the female animals “focus” on certain types of wood while they were in heat in order for them to give birth to issue that were similar to those blocks of wood.
 Mishneh L'Melech to Laws of Forbidden Sexual Relations 21:18
 See Maimonides, Laws of Forbidden Sexual Relations 21:8
 Cited in Tosafos to Yevamos 66a
 To Yevamos 66a
 In his pseudonymous responsa Torah Lishmah §502, under the penname "Ezekiel the Blue"
 Kiryas Sefer to Maimonides' Laws of Forbidden Sexual Relations, Chapter 21
 Leviticus 18:3
 This idea is also quoted in the Talmud: Yevamos 62a, Ibid. 63b, Niddah 13b, and Avodah Zarah 5a.