Gemara

5b - Question: Why can't a Sotah do Yibum?

6a - Answer (Rava): A Kal v'Chomer teaches this. If she became forbidden to the one she was permitted to (i.e. her husband), all the more so she becomes (permanently forbidden) to the one she was forbidden to (the Yavam).

Question (Abaye): If a Kohen's wife was raped and his brother is a Chalal, she should not do Yibum, for she became forbidden to her husband!

Answer: If a Yisrael's wife was raped, she is not forbidden. A Chalal is like a Yisrael, so regarding him she did not become forbidden, so the Kal v'Chomer does not apply.

27b (Mishnah): Just like she is forbidden to her husband, she is forbidden to the Bo'el. It says "Nitme'ah (she was defiled)", and "v'Nitme'ah."

26a (Mishnah): She may be warned about seclusion with any of the Arayos...

One might have thought that "Nitme'ah... v'Nitme'ah" forbid her to her husband and the Bo'el only when divorce or her husband's death would have permitted her to the Bo'el, if not for the Bi'ah. The Mishnah teaches that this is not so.

Yevamos 56a (Rav Amram): If a Yisrael's wife was raped, even though she is permitted to her husband, she is disqualified to Kehunah.

68a (Rav Yehudah): "U'Vas Kohen Ki Sihyeh l'Ish Zar" - once she has Bi'ah with a man forbidden to her, he disqualifies her (from Terumah).

Rishonim

Rambam (Hilchos Sotah 2:12): Any woman who was warned and secluded and did not drink for any reason is permanently forbidden to that man. We have a tradition: just like she is forbidden to her husband, she is forbidden to the Bo'el.

Rosh (Yevamos 6:6): If a Yisrael's wife was raped, even though she is permitted to her husband, she is disqualified to Kehunah. It says three times in the Parshah "Nitme'ah." The first forbids her to her husband. It applies to "v'Hi Lo Nitpasah" to permit when she was raped. It does not say "v'Hi Lo Nitpasah" regarding the latter two, which forbid to the Bo'el and Kehunah. Regarding Kehunah, Ones and Ratzon are the same. However, it is unreasonable to forbid to the Bo'el when she is permitted to her husband. Alternatively, the Isur to Kehunah is due to "U'Vas Kohen..."

Birkei Yosef (EH 11:4): If the only reason to permit to the Bo'el is that it is unreasonable to forbid to him when she is permitted to her husband, a Kohen's wife is forbidden also to the Bo'el. However, if the Isur Kehunah is learned from "U'Vas Kohen...", the entire Parshah of "Nitme'ah" refers to when she was to forced. If so, there is no source to forbid to the Bo'el when she was forced. It seems that the Ramban, Ro'oh, Rosh Rashba hold like this.

Ri (in Tosfos Yevamos 35a DH Af): "V'Hi Lo Nitpasah", which distinguishes Ones from Ratzon, applies only to the Isur to her husband. It does not apply to the Isur to Kehunah. Regarding the Isur to the Bo'el, we can distinguish regarding a Yisrael's wife. It is reasonable that she is forbidden to the Bo'el only when she is forbidden to her husband. Therefore, if he was Mezid and she was Shogeg, she is permitted to the Bo'el.

Mishneh l'Melech (DH Od Yesh Li): If Reuven raped a Kohen's wife, is she forbidden to Reuven because she is forbidden to her husband? Or, perhaps this applies only to a Yisrael's wife forbidden because she was Mezid! We can infer from the Ri that she is forbidden. Chelkas Mechokek (EH 11:10) was unsure about this.

Rebuttal (Birkei Yosef 4 DH uv'Matu): Perhaps the Ri was meticulous to say that "V'Hi Lo Nitpasah" does not apply to the Isur to Kehunah. The Ri considered whether it applies to the Isur to the Bo'el, and reasoned that it does, for it is unreasonable to forbid a Yisrael's wife to the Bo'el when she is permitted to her husband. If so, also regarding a Kohen's wife we permit when she was forced.

Hagahos R. Baruch Frankel (EH 11:11, Rosh Pinah edition): Even according to the Mishneh l'Melech, we can ask about one who raped a Kohen's wife who was already forbidden to her husband. One who was Mezanah twice is forbidden to the second Bo'el (Sanhedrin 41a); perhaps Ones is more lenient.

Poskim

Shulchan Aruch (EH 11:1): If Levi was suspected of Bi'ah with a married woman, if there was warning and seclusion and she did not drink Mei Sotah, since she became forbidden to her husband, she is permanently forbidden to Levi.

Rema: Similarly, if she was forbidden to her husband due to Levi, she is forbidden to Levi.

Beis Shmuel (3): What does the Rema add to the Mechaber? It seems that he teaches about a Kohen's wife who was raped. We hold that when she is forbidden to the Bo'el, she is forbidden to her husband, and vice-versa.

Rebuttal (Dagul me'Revavah): We learned the other direction, just like she is forbidden to her husband, she is forbidden to the Bo'el! According to the Beis Shmuel, if a Kohen raped a Yisrael's wife, she is forbidden to her husband. We never heard such a law!

Correction (Hagahos Ma'aseh Nisim 3): R. Baruch Frankel says that the text of the Beis Shmuel should say 'just like she is forbidden to her husband, she is forbidden to the Bo'el'. The words 'and vice-versa' do not belong here, rather, later, like in the first edition of the Beis Shmuel.

Shirei Korban (Sotah Sof 21a): The text of the Rema should say 'similarly, if she was not forbidden to her husband, she is forbidden to Levi', i.e. when a Yisrael's wife was Shogeg and Levi was Mezid, like the Yerushalmi.

Pischei Teshuvah (5): If we will correct the text, presumably we should say that the Rema teaches like Be'er Heitev (4) wrote, that even a second Bo'el is forbidden. The Mechaber wrote that the Bo'el is forbidden because the husband became forbidden due to him.