Normal sexual intercourse is vaginal, but some people desire anal intercourse, which the Talmud calls bi’a she-lo ke-darka, “abnormal intercourse.” Clearly, if anal sexual intercourse is painful for the wife or done against her wishes, it is forbidden. But what if she consents or even desires it? The Talmud records (Nedarim 20b) that the Sages ruled that it is not prohibited. On the other hand, we learn (Yevamot 34b) that the sin of Yehuda’s sons, Er and Onan, was that they penetrated Tamar anally, thereby wasting their seed. This was evil in God’s eyes (Bereishit 38:7 and 38:10), and He put them to death.
Most Rishonim explain that when a couple engages in anal intercourse in order to avoid pregnancy, it is considered a waste of seed and is forbidden. However, if they engage in it occasionally, it is not prohibited. Some Rishonim explain that the Sages permitted anal sexual intercourse as long as the husband does not ejaculate in the anus. Rather, the couple must later engage in vaginal intercourse, culminating in ejaculation. Some halakhic authorities forbid even this.
In practice, a man who feels a need may rely on the opinion of most poskim, who permit this on occasion, as long as his wife consents.
Some say that even vaginal intercourse should take place with the man on top of the woman, face to face (i.e., the missionary position). Some are very insistent on this. Although there is something virtuous about this, technically all possible positions are permitted, as long as both spouses desire them. If changing position enhances the pleasure of either spouse, then doing so is a fulfillment of the mitzva of simḥat ona. However, if neither spouse wants to change, it is preferable for their sexual relations to take place in the optimal manner, namely the missionary position. Even if they find a different position more enjoyable, when they are trying to conceive it is better to use the missionary position.
This seems to contradict a talmudic passage (Yevamot 34b), which states that the sin of Er and Onan was having anal intercourse with Tamar, thereby wasting their seed. Most poskim limit this prohibition to doing this regularly to avoid conception. When done occasionally, it is permissible. This is the opinion of Tosafot (Sanhedrin 58b, s.v. “mi”); Tosafot Rid (Yevamot 12b, s.v. “tanei”); Or Zaru’a (Kuntres Ha-re’ayot on Sanhedrin 58a); Rosh (Yevamot 3:9); Rabbeinu Yeruḥam (Toldot Adam Ve-Ḥava, netiv 23, part 1); Mordechai (Shevu’ot, Hilkhot Nidda §732); Hagahot Maimoniyot (Laws of Sexual Prohibitions 23:4); Ritva (in his primary explanation as cited in Shita Mekubetzet, Nedarim 20b); and Rabbeinu Yona (Sanhedrin 58b). This is also the correct text of Rambam (MT, Laws of Sexual Prohibitions 21:9). It is also the opinion of Yam Shel Shlomo (Yevamot 3:18); Levush (240:14); Shetilei Zeitim (240:20); and Torot Emet, Yeshu’ot Yaakov, and Erekh Shai in their commentaries on SA EH 25:2.
Others are stringent and maintain that anal sexual intercourse is permitted only on condition that the husband does not ejaculate there. This is the opinion of R. Avraham Min Ha-har (Nedarim ad loc.); Orḥot Ḥayim (Hilkhot Ketubot §7); Ri (in his first explanation as cited in Tosafot, Yevamot 34b s.v. “ve–lo”); Beit Yosef (EH 25:2); and AHS (EH 25:11). This is also the reading that appears in the printed editions of Rambam’s Mishneh Torah. Some are even stricter and do not permit anal penetration at all (Sefer Ḥaredim ch. 64; Shlah, Sha’ar Ha-otiyot, Kedushat Ha-zivug §§360-364). They understand the phrase “shelo ke-darka” (non-normal) to mean that the wife is on top of her husband or that the husband is behind his wife during vaginal intercourse. Even when it comes to these positions, people who do not feel a need for them are considered holy. According to these authorities, the Sages did not even speak of anal intercourse because it is forbidden. However, this interpretation is rejected by almost all poskim. Rema mentions the first two opinions and concludes, “Even though this is all permitted, anyone who sanctifies himself [by refraining from] what is permitted to him is considered holy” (EH 25:2).
In practice, the halakha follows those who are lenient, as they are in the majority. Furthermore, according to most poskim the entire discussion relates to a rabbinic prohibition (because it is not truly a violation of the prohibition against wasting seed, as masturbation would be for a man). On the contrary, if this brings the husband joy and satisfaction, it has value and is a mitzva. Certainly, then, if both husband and wife enjoy it, it is a fulfillment of the mitzva of ona according to most poskim. Rav Kook wrote similarly in his explanation of the lenient opinion: “Even in this manner, when done occasionally, since the Torah permits it according to this opinion, it is considered something of a mitzva as it helps satisfy the husband, and therefore is not really a waste [of seed]” (Ezrat Kohen §35).
. According to Kalla Rabbati 1:23, “He on the bottom and she on top – this is impudent.” Several Rishonim cite this (Raavad, Sha’ar Ha-kedusha; Ohel Mo’ed; Sefer Ha-eshkol; Menorat Ha-ma’or; and Tur). SA 240:5 cites it as well, as do many Aḥaronim. Sefer Ḥasidim §509 limits the admonition to mikveh night, when it is more likely that the woman will conceive. This is also the approach of Birkei Yosef 240:7 and Da’at Torah 240:5.
In contrast, most Rishonim, and Rambam foremost among them, do not record the admonition at all. As Rabbeinu Yeruḥam writes, “The case in Nedarim about a woman who said that she set a table and her husband turned it over, which the Sages ruled was permitted, was referring to anal intercourse. It does not mean that she was on top and he was beneath her, because that is certainly permitted, and she would not have been upset about it” (Toldot Adam Ve-Ḥava, netiv 23, part 1). Zohar (II 259a) implies that the admonition is against positions where the husband faces his wife’s back, “For it says, ‘And he shall cling to his wife’ (Bereishit 2:24) – specifically to his wife, not behind his wife.” Ma’amar Mordechai 240:7 explains that the problem with the “impudent” position is that it seems like the woman is trying to dominate her husband, implying that if both spouses consent, it is permissible. In practice, since according to most poskim it is not forbidden to change from the missionary position, they may. Even those who are stringent view insistence on the missionary position as a pious practice, not a requirement, and some explain that the stringency applies only when there is no mutual consent. Therefore, a couple who wants to change from the missionary position may do so, and it is even a mitzva if it increases their pleasure. Nevertheless, when a couple is hoping to conceive, the pious practice is for them to defer to the stringent opinion and have sexual relations in the missionary position.