Gemara

27b (Mishnah - R. Akiva): Just like she is forbidden to her husband (until she drinks), she is forbidden to the Bo'el - it says "Nitme'ah" and "v'Nitme'ah".

R. Yehoshua: Zecharyah ben ha'Katzav also expounded this way.

Rebbi says, it says "Nitme'ah" twice. This teaches that she is (permanently) forbidden to her husband and the Bo'el.

28a: Rebbi expounds the repetition of "Nitme'ah (she was defiled)". R. Akiva expounds the extra 'Vov' in "v'Nitme'ah."

29a: R. Akiva expounds four verses. They teach the Isurim to her husband, the Bo'el, Terumah, and Kehunah.

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

One might have thought that "Nitme'ah... v'Nitme'ah" forbids 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.

Kesuvos 9a - Question: Why didn't Bas Sheva become forbidden (to David)?

Answer #1: She was forced.

Answer #2 (Rav Shmuel bar Nachmani): Everyone who went to war in David's army divorced his wife.

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

95a: If a man divorced his wife (a Sotah), or said that he will not give her to drink, she is (permanently) forbidden to the Bo'el.

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.

Tosfos (Shabbos 56a DH Likuchin): Even if Bas Sheva had not been divorced and David sinned with her, she would be permitted to him. She was forced, so she was permitted to her husband, and hence also to the Bo'el!

Mishneh l'Melech: If a Yisrael's wife was Mezanah b'Ones or b'Shogeg (she was forced or unaware), she is permitted to her husband. I am unsure whether she is permitted to the Bo'el (after divorce) if he was Mezid. If the Isur to the Bo'el depends on the Isur to her husband, she is permitted. Answer #1 in Kesuvos permits. It is not clear whether Answer #2 agrees. Tosfos permits her. Similarly, if she was Mezid she is forbidden to him, even if he was Shogeg. The Yerushalmi (Sotah 21a) discusses when the water checked (started to kill) the Bo'el, but not her. The one who holds that when merit suspends death, it is apparent (immediately that she will die) must say that he was Mezid and she was Shogeg. If the water checked her, but not him, she was Mezid and he was Shogeg. In this case, she is forbidden to her husband and the Bo'el. If witnesses said that he was Mezid and she was Shogeg, surely she is permitted to her husband and forbidden to the Bo'el. Why did the Rishonim ignore this Yerushalmi? One could explain the Yerushalmi differently, to forbid the Bo'el only when she was Mezid.

Korban ha'Edah (Sotah Sof 21a): The Yerushalmi learns from "Letam'ah Vah" that it depends on her: whenever she was not Mezid, there is no Isur.

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 Isur to the Bo'el. However, it is unreasonable to forbid to the Bo'el when she is permitted to her husband.

Rosh (Teshuvah 32:15): She is forbidden to the Bo'el only when she is forbidden to her husband. When there are no witnesses, we do not forbid her to her husband, but if the Bo'el knows that he had Bi'ah with her, he knows that he is forbidden. Tosfos proves this from Kesuvos 9a. Surely, no witnesses saw David's Bi'ah with Bas Sheva. Still, the Gemara asked why he was not forbidden to her.

Nimukei Yosef (Yevamos Sof 29a): If a Yevamah was Mezanah with a stranger, she is permitted to her Yavam, for he did not sin. The Ritva says that we fine her, and forbid her to the stranger (even after Chalitzah).

Mishneh l'Melech (ibid.): Does the Nimukei Yosef always fine to forbid the Bo'el even when she is permitted to her husband, e.g. she was forced? Perhaps this decree was after the time of David. Or, perhaps we should have decreed to forbid the Yevamah to the Yavam, for this is like Nisu'in, just we did not decree about a Yavam for it is not common. I do not know his source for this decree. It is difficult to say that he made a decree not in the Gemara.

Chasam Sofer (EH 1:26 DH umi'Kol): If a married woman b'Mezid enticed a man to have Bi'ah with her (he was Shogeg), he is forbidden mid'Oraisa. The Yerushalmi says that it would be a mockery to permit him when she was Shogeg and he, b'Mezid, enticed her! This is the Nimukei Yosef's source

Beis Shmuel (EH 159:9): It is difficult to say that the decree was enacted after David. The primary answer is that Bas Sheva was already divorced. Similarly, one who raped a married woman is permanently forbidden to her.

Avnei Nezer (EH 2:235:4): I say that the Nimukei Yosef forbids lest a sinner profit. Normally, a Yavam will not want to do Yibum with a Yevamah who was Mezanah, and he will do Chalitzah. The Beis Shmuel extends the concern to a married woman, for a man is apt to divorce his wife after she was raped.

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.

Chelkas Mechokek (1): This is from the Rambam. What is the source? If she claims that she is Tahor and wants to drink, granted, her husband can forbid her to himself, but how can he forbid her to others?

Mishneh l'Melech (Sotah 2:12 DH Kol and Gra 2): Seclusion forbids her to her husband, the Bo'el and Terumah. If he does not want her to drink, she never drinks, so she is forbidden forever. Yevamos 95a proves this.

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

Chasam Sofer (EH 1:26): Rebbi expounds the repetition of "Nitme'ah", so the Isur to the Bo'el does not depend on the Isur to her husband. It applies even if she was forced. The Halachah follows R. Akiva, R. Yehoshua and Zecharyah ben ha'Katzav, who expound the extra 'Vov'. Therefore, she is forbidden to the Bo'el only when she is forbidden to her husband.

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.