1) THE WOMAN'S PROHIBITION TO THE ADULTERER
OPINIONS: The Mishnah teaches that just as the Sotah is prohibited to her husband, she is prohibited to the suspected adulterer ("Bo'el"). Rebbi Akiva derives this prohibition from the "Vav" of the word "v'Nitma'ah" (Bamidbar 5:29). Rebbi Yehoshua derives it from the fact that the Torah repeats the word "Nitma'ah" (see Gemara, 29a).
Why does the Mishnah compare the woman's prohibition to the Bo'el with her prohibition to her husband ("just as she is prohibited to her husband, she is prohibited to the Bo'el")? The Mishnah should simply state that "the woman is prohibited to the husband and to the Bo'el." What is the point of the comparison? Does the Mishnah mean to teach that the reason she is prohibited to the Bo'el is that he caused her to become prohibited to her husband, and therefore he deserves to become prohibited to her as well (so that he not be a "Chotei Niskar," a sinner who benefits as a result of his sin)? Alternatively, perhaps the Mishnah simply means that the same type of circumstances which can create a prohibition to the husband are severe enough to create a prohibition to the Bo'el as well.
The MISHNEH L'MELECH (Hilchos Sotah 2:12) discusses this question at length. He points out that there are a number of practical differences between these two possible intents of the Mishnah. One practical difference exists in the case of a woman who has relations with the adulterer b'Shogeg (she thought he was her husband) or b'Ones (she was forced), while the adulterer acted b'Mezid (intentionally). The Halachah is that the wife of a Yisrael is permitted to remain married to her husband when her extramarital relations were b'Shogeg or b'Ones (see Gemara, end of 28a). In such a case, is she also permitted to the Bo'el as well (after her husband divorces her or dies)? According to the first way of understanding the prohibition to the Bo'el, she should be permitted to him because he did not succeed in prohibiting her to her husband. According to the second approach, she should be prohibited to the Bo'el because the woman's status to her husband has no bearing on her status to the Bo'el.
A similar practical difference exists in the opposite situation, in the case of a woman who sinned b'Mezid (intentionally) while the Bo'el's act was b'Shogeg or b'Ones. According to the first approach, the woman might be prohibited to him since the Bo'el's act causes her to become prohibited to her husband, while according to the second approach, she should be permitted to the Bo'el because his sin was not so severe.
(a) The Gemara in Kesuvos (9a) asks how David ha'Melech was permitted to marry Bas Sheva. Since a Sotah is prohibited to the Bo'el, she should have been prohibited to David ha'Melech. The Gemara first answers that Bas Sheva was an Anusah since she could not refuse the king, and an Anusah is not prohibited to the Bo'el. The Gemara then answers that she was permitted to him for a different reason: Uriyah had given her a conditional document of divorce when he went to war, and thus she was not a Sotah at all.
The Mishneh l'Melech points out that it is clear that the first answer of the Gemara supports the first explanation proposed above: the woman is prohibited to the Bo'el only if he caused her to become prohibited to her husband. Therefore, if the Bo'el acted b'Mezid and the woman b'Ones, she is permitted to the Bo'el. Although it is possible that the second answer of the Gemara rejects that view, TOSFOS in Shabbos (56a, DH Lekuchin) implies that both answers in the Gemara agree with that view.
Further support for this view may be found in TOSFOS in Yevamos (3b, DH l'Fi). The Gemara there states that although the Tzarah of a Sotah does not perform Yibum or Chalitzah when the husband dies, she is not included in the Mishnah's list in the beginning of Yevamos because no case can be construed in which she would have a "Tzaras Tzarah" (because when a woman is a Sotah, none of the brothers of the deceased husband are permitted to her or to her Tzaros). Tosfos asks that there is a case of "Tzaras Tzarah" -- in a situation in which the Sotah later married the brother of the Bo'el, and upon his death her Tzarah (the other wife of the Bo'el's brother) did Yibum with another brother of the Bo'el. The second brother's other wife is thereby rendered a "Tzaras Tzarah."
Tosfos answers that no case can be construed in which the husband will be prohibited to the Tzaras Tzarah of the Sotah. Since the prohibition to the Bo'el is connected to the prohibition to the husband, the "Tzaras Tzarah" will not be prohibited to the Bo'el either.
Tosfos implies that the reason for the prohibition to the Bo'el is the prohibition he caused for the husband. Accordingly, the Bo'el is not prohibited in any manner in which he did not prohibit the husband (as the KOVETZ HE'OROS writes in Yevamos 12:6). How, though, can this be reconciled with the Gemara earlier (25a) which says that a man who is prohibited to his wife because of an Isur Lav may do Kinuy in order to prohibit her to the Bo'el when she does Setirah? Since she is already prohibited to her husband and the Bo'el did not create that Isur, why should the Bo'el be prohibited to her?
The answer is that even in such a case, the Bo'el did make the woman prohibited to her husband even though she was prohibited to her husband because of a pre-existing Isur. The Gemara in Yevamos (32b) teaches that although there is a rule that "Ein Isur Chal Al Isur" -- one Isur cannot take effect when there is already an Isur in effect, the second Isur does amplify the prohibition; it does not take effect only with regard to punishment (such as to require that the transgressor receive an additional set of Malkus). Hence, since the Bo'el did create an Isur, he becomes prohibited to the woman.
(b) However, TOSFOS here (DH k'Shem) cites the Yerushalmi which says that if the Bo'el sinned b'Mezid and the woman sinned b'Shogeg, although she is permitted to her husband, the Bo'el still becomes prohibited to her.
The Yerushalmi apparently maintains that the Isur of the Bo'el is independent of whether he caused the wife to become prohibited to her husband.
However, the Yerushalmi continues and says that even if she sins b'Mezid and the Bo'el sins b'Shogeg, the Bo'el becomes prohibited to her since his act made her prohibited to her husband! The Yerushalmi clearly contradicts itself.
Apparently, the Yerushalmi maintains that there are two possible reasons for why the Bo'el becomes prohibited to the woman: he becomes prohibited to her because he prohibited her to her husband, or -- even if he did not prohibit her to her husband -- since he transgressed a serious violation of the Isur of Eshes Ish he becomes prohibited to her because of the Aveirah he did.
With regard to David ha'Melech, the Yerushalmi must understand that David ha'Melech was permitted to Bas Sheva because of the Get she received when her husband to war.
HE'OROS B'MASECHES SOTAH (in the name of Rav Elyashiv shlit'a) cites the CHASAM SOFER (EH 26) who suggests that the question of the Mishneh l'Melech may depend on the Machlokes between Rebbi Yehoshua and Rebbi Akiva in the Mishnah. Rebbi Akiva, who derives the Isur to the Bo'el from the letter "Vav" of "v'Nitma'ah," may understand that the Isur to the Bo'el is a corollary of the Isur to the husband (the Isur to the husband is the subject of the word "Nitma'ah" to which the "Vav" is appended). Rebbi Yehoshua, who argues with Rebbi Akiva and derives the two Isurim from two separate words, may rule like the Yerushalmi that the Isur to the Bo'el may apply even when there is no Isur to the husband.
Another question that arises is whether a man who rapes the wife of a Kohen becomes prohibited to her, since the act causes her to become prohibited to her husband who is a Kohen. (The Gemara in Yevamos (56b) explains that "v'Nitma'ah," the Isur of Tum'ah, applies to the wife of a Kohen even when he act was done b'Ones.) According to the Yerushalmi which says that even one who lives with the wife of a Yisrael (where the woman's act was b'Shogeg) becomes prohibited to her, it is obvious that one who rapes the wife of a Kohen becomes prohibited to her. According to the Bavli, however, which permits the Bo'el to the wife of a Yisrael whom he raped, what is the Halachah in the case of the wife of a Kohen? Since he prohibited her to her husband, is he prohibited to her, or does the Derashah teach that the Bo'el is prohibited to the woman only when he prohibits the woman to her husband as a result of her sin being willful and intentional?
The Mishneh l'Melech (ibid.) discusses this question. He cites the CHELKAS MECHOKEK (EH 11:3) who prohibits the Bo'el to the wife of a Kohen whom he raped. The Mishneh l'Melech cites support for this ruling from the words of TOSFOS in Yevamos (35a, end of DH Af Al Pi).