Gemara

(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 secluding with any of the Arayos...

Objection: This is obvious!

Answer: It says "Nitme'ah" and "v'Nitme'ah", to teach that she becomes forbidden to her husband and the Bo'el. One might have thought that this applies only when she would have been permitted to the Bo'el (if she were widowed or divorced) if not for the adultery. The Mishnah teaches that this is not so.

26b: Rav Hamnuna taught that one can warn about seclusion with a Nochri.

She is forbidden to her husband and the Bo'el due to "Nitme'ah", and "v'Nitme'ah." One might have thought that this excludes a Nochri, who was forbidden to her before the Bi'ah. Rav Hamnuna teaches that this is not so.

Kesuvos 3b (Beraisa): At times of danger, virgins married on Tuesday.

(Rava): The danger was a decree that a virgin married on Wednesday must first have Bi'ah with the (Nochri) mayor. It is called danger (not Ones), because there are virtuous women who would resist, and be killed for doing so. We cannot publicize that Ones is permitted, for there are Prutzos (immoral women).

26b (Mishnah): If Nochrim took a woman captive for money (they intend to charge ransom to release her), she is permitted to her husband. If she was taken for a capital offense, she is forbidden to her husband.

Megilah 15a (R. Aba): "V'Cha'asher Ovadti Ovadti" - just like I (Esther) was lost from my father's house (orphaned), I am lost to you (her husband Mordechai, for now she went willingly to Achashverosh).

Yevamos 95a (Beraisa): A light Isur (Zenus of a married woman) causes the Oser (one who forbids, i.e. her husband forbids her to other men) to be forbidden.

Rishonim

Rambam (Hilchos Sotah 2:12): If any woman was warned and secluded and did not drink Mei Sotah, whether her husband did not want her to drink or she did not want to, or a witness of Tum'ah came, or she admitted, or she was one of the women who does not drink, or Beis Din warned her, since she is forbidden to her husband, she is permanently forbidden to the man she was secluded with, just like to her husband. If he transgressed and married her, we force him to divorce her, even if he had children from her. A tradition teaches that just like she is forbidden to her husband, she is forbidden to the Bo'el.

Rosh (Kesuvos 1:4): A case occurred in which Leah became an apostate and had Bi'ah with a Nochri. She repented, and the Nochri converted. R. Tam permitted them to marry. 'She is forbidden to her husband and the adulterer' does not apply to Bi'as Nochri, which is like Bi'ah with an animal.

Rebuttal (Rivam, cited in Rosh): Our Gemara said that there are Prutzos (they would willingly have Bi'ah with the Nochri, and be forbidden to their husbands.) Also, a woman taken captive (for a capital offense) is forbidden to her husband, lest she willingly had Bi'ah with her captors, to dissuade them from killing her. Esther became forbidden to Mordechai when she went willingly to Achashverosh. These show that Bi'as Nochri forbids to her husband.

Question (Korban Nesan'el 8): Bi'ah with a married woman forbids the Oser (her husband - Yevamos 95a). Her husband does not forbid a Nochri (who is forbidden in any case), so a Nochri's Bi'ah does not forbid to her husband!

Rosh: Since Bi'as Nochri forbids her husband, it also forbids the Nochri. This is why one may warn against seclusion with a Nochri. 'We should publicize that Ones is permitted' means that she need not forfeit her life, for she is passive. However, R. Tam's law is correct for a different reason. "Nitme'ah" and "v'Nitme'ah" forbid her to her husband and the Bo'el, i.e. when the Bi'ah forbids her to the Bo'el. This does not apply to a Nochri, for she was already forbidden. There was a Hava Amina that since the Bi'ah does not forbid her to a Nochri, it does not forbid her to her husband. Rav Hamnuna rejected this. The Bi'ah forbids her to her husband, even though it does not add an Isur to the Nochri.

Mordechai (Sanhedrin 720): R. Yechiel permitted her to the Nochri after conversion, for a convert is like a new person.

Terumas ha'Deshen (2:29): The Mishnah said that one can warn against Arayos. We asked and answered like about Rav Hamnuna's law. If so, the Rosh should likewise permit the Bo'el (if the Isur Ervah goes away)! Why did the Gilyon (it seems that he refers to Shitah Mekubetzes Kesuvos 3b DH v'Chasuv) seek a Heter because a convert is like a new person? This shows that the Gilyon did not accept the Rosh's proof. 'She becomes forbidden to the Bo'el' applies to one's wife's sister, i.e. to forbid after his wife will die. It does not apply to a Nochri, for after conversion he is like a new person. The Mishnah discusses Arayos that are permanently forbidden. However, we cannot say that a Nochri, who needs an act to be permitted (conversion), is unlike a wife's sister, who is permitted automatically through death. Also death is not considered an act (Kidushin 62a)!

Rambam (Hilchos Gerushin 10:14): If a Nochri had Bi'ah with a Yisraelis, he may not marry her, even if he converts. If he did, we do not force him to divorce her.

Hagahos Maimoniyos (2): This is like the Ri and Rivam. However, it seems that they force him to divorce her if he married her. Rather, the Rambam forbids even if she was single, lest people slander them.

Poskim

Shulchan Aruch (19): Some say that if a married woman had Bi'ah with a Nochri and was divorced and the Nochri converted, she is permitted to him, for 'she is forbidden to her husband and the Bo'el' does not apply to Bi'as Nochri.

Beis Yosef (DH uv'Orchos): Orchos Chayim says not to rely on R. Tam, for all Chachamim of his generation disagreed.

Darchei Moshe (6): The Mordechai agreed with R. Tam.

Rema: In any case, if Levi was Mezaneh with his wife's sister Dinah and Dinah's husband divorced her, Levi is forbidden to Dinah even after his wife dies, even though he was already forbidden to her when they were Mezaneh.

Beis Shmuel (EH 178:13) Kidushin is possible only after conversion, and a convert is like a new person. Kidushin is possible with one's wife's sister, i.e. after his wife will die.

Note: The other Meforshim discuss Heter Bi'ah, not Kidushin. However, Tosfos (Yevamos 16b DH ka'Savar) holds that mid'Oraisa, Bi'as Nochri is permitted in private. It is forbidden only in public (Kana'in Pog'in Bo).