Gemara

(R. Elazar): A woman becomes forbidden to her husband only through Kinuy (warning not to be secluded with a particular man) and seclusion, like the episode (of David and Bas Sheva).

Objection: This cannot be correct. There was no Kinuy and seclusion in that episode! Also, she did not become forbidden!

Correction: R. Elazar meant, we learn that a woman becomes forbidden only through Kinuy and seclusion from the episode, in which there was not Kinuy and seclusion, and she did not become forbidden.

Question: Do only Kinuy and seclusion forbid, but not witnesses (of Bi'ah)?!

Answer: Rather, a woman becomes forbidden through two witnesses. If there were (two witnesses about) Kinuy and seclusion, even one witness (of Bi'ah forbids her permanently, and she may not drink the Sotah water);

Sotah 31a (Mishnah): If (we know that there was Kinuy and seclusion, and) one witness testifies that she had Bi'ah, she does not drink. Even a male or female slave is believed, even to deprive her of her Kesuvah;

(Any of the women suspected of hating her, i.e.) her mother-in-law... are believed to prevent her from drinking, but not to deprive her of her Kesuvah.

31b (Ula): Wherever the Torah believed one witness, he is believed like two.

2a - Question: What is the source that the Torah believes one witness here?

Answer (Beraisa): "V'Ed Ein Bah (and she was not forced)" - there are not two witnesses (about Tum'ah, only one, and she is forbidden)!

Suggestion: Perhaps it means that not even one witness knows about her!

Rejection: "One witness will not rise against a man". We already know that the verse discusses one witness, for "Yakum" (rise) is singular. The Torah specified "one" to teach that in the Torah "Ed" always refers to two witnesses, unless the Torah specifies 'one'.

Summation of answer: The Torah said that there are not two witnesses, only one, and if she was not forced, she is forbidden.

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

Version #1 (Rabah): If a Kohen's wife was raped, her husband is lashed for Bi'ah with her (after this), since she is a Zonah.

Question: Is he lashed for Zonah, but not for Tum'ah ("Acharei Asher Hutama'ah")?

Answer: Rather, he is lashed even for Zonah.

Version #2 (Rabah): If a Kohen's wife was raped, her husband is lashed for Bi'ah with her, for Tum'ah.

He is lashed for Tum'ah, but not for Zonah. This shows that she does not become a Zonah when forced.

Rishonim

Rif and Rosh (Kidushin 29a and 3:15): A woman becomes forbidden to her husband only through two witnesses. If there were Kinuy and seclusion, even one witness forbids her.

Rambam (Hilchos Isurei Bi'ah 1:22): If a Kohen warned his wife not to be secluded with Ploni and she transgressed and one witness says that there was Bi'ah, if her husband had Bi'ah with her afterwards he is lashed for Zonah.

Rebuttal (Ra'avad): She is lashed only for Tum'ah. Zonah does not apply b'Ones, according to the latter version of Rabah.

Magid Mishneh: Surely, the Halachah follows Chachamim (Yevamos 11b) who say that there is a Lav "Acharei Asher Hutama'ah" for a Sotah who had Bi'ah. The Rambam holds that a Yisrael is not lashed for this, for the verse primarily forbids Machazir Gerushaso after she remarried. It also forbids Sotas Vadai. One is not lashed for Sotah because it is Lav shebi'Chlalos i.e. the Lav forbids two things. The Rambam exempts from lashes in such cases. One is lashed for Machazir Gerushaso because this is the simple meaning. The Rambam holds that Zonah applies even b'Ones. Here he teaches that one witness establishes her to be a Zonah. Both versions of Rabah obligate lashes for Tum'ah. The Rambam holds that the Sugya holds like the opinion that lashes for Lav shebi'Chlalos, but we do not hold like it. It is not clear why the Ra'avad assumes that here the Bi'ah was Ones, and why he rules like the latter version of Rabah, which is unlike Rav Sheshes.

Migdal Oz: One witness forbids her, but she does not lose her Kesuvah. The Mishnah in Sotah explicitly says that even a slave prevents her from drinking, but she does not lose her Kesuvah.

Note: Our text of the Mishnah says that anyone is believed to make her lose her Kesuvah except for the five women who are prone to hate her.

Avnei Nezer (EH 201 18) The Rambam holds that one witness is believed only to prevent her from drinking. Her original Isur remains, the Aseh "V'Nitme'ah". We learn that two witnesses are needed for Devar sheb'Ervah from Sotah, so he is not believed about Tum'ah. However, the Rambam holds that Zonah is not Devar sheb'Ervah, so we believe the one witness regarding the Isur Zonah.

Rambam (Hilchos Gerushin 11:14): Included in this Lav (of Machazir Gerushaso, "Lo Yuchal Ba'alah...") is a married woman who was Mezanah. She is forbidden to her husband, and he is lashed for Bi'ah with her. It says "Acharei Asher Hutama'ah", and she became Teme'ah. The only exception is a Yisrael's wife who was raped. Therefore, if a woman was forbidden through Kinuy and seclusion, if her husband has Bi'ah with her he gets Makos Mardus.

Question: Here the Rambam obligates lashes for "Acharei Asher Hutama'ah". In Hilchos Ishus he exempts!

Answer #1 (Kesef Mishneh and Lechem Mishneh): The Rambam exempts mid'Oraisa due to Lav shebi'Chlalos. 'He is lashed' refers to Makos Mardus for Tum'ah (even when she was definitely Mezanah). He cites the Gemara verbatim, even though 'he is lashed' is misleading.

Answer #2 (Kesef Mishneh): Some say that here the Rambam teaches that if she was Mezanah and her husband divorced and remarried her he is lashed even if she did not marry anyone else in between. This is not Lav shebi'Chlalos; it is the same Lav of Machazir Gerushaso.

Answer #3 (Lechem Mishneh): In Hilchos Ishus the Rambam discusses when there is one witness of Tum'ah. One witness makes her a Zonah, but he does not suffice to obligate lashes for Tum'ah. However, the Magid Mishneh brought good proofs that the Rambam exempts for Tum'ah mid'Oraisa.

Note: The Rosh (Yevamos 1:4) says that there is a Lav of Tum'ah for a Sotas Vadai, but not for a Safek. The Gemara says that the Torah believes one witness to say that she had Bi'ah. Perhaps this refers to forbidding her to drink, but he is not believed regarding lashes for Tum'as.

Poskim

Shulchan Aruch (EH 178:14): If one witness testified that she had Bi'ah with the man she was warned about, she is divorced without a Kesuvah.

Beis Yosef (DH Ba): One witness is believed to make her lose her Kesuvah because there are Raglayim l'Davar (grounds to believe) that she had Bi'ah.

Beis Shmuel (11): Why does the Shulchan Aruch discuss one witness? Nowadays no Sotah drinks, and she becomes forbidden and loses her Kesuvah through seclusion itself! The Rambam and Tur say that after one witness testified that she is Teme'ah, we do not heed a contradictory witness.

Beis Meir (DH Afilu): Perhaps the Shulchan Aruch teaches that even without Kinuy and seclusion, if there is Raglayim l'Davar we believe one witness, at least to make him divorce her.

Question (R. Akiva Eiger Mishnayos Sotah 6:2): The Mishnah believes one of the women suspected of hating her to prevent her from drinking, but not to deprive her of her Kesuvah. Since she does not drink, the Safek remains, so she should not get her Kesuvah! Also the Yerushalmi asked why she loses her Kesuvah through one witness. It answered that the Torah believed one witness like two. Why is this necessary? Perhaps when she does not drink if she seized her Kesuvah she would keep it, for it is a Safek and she has a Vadai claim. We believe one witness to say that she is Vadai Teme'ah, unless the witness is one of the women who hate her. The Rambam (Sotah 1:20) connotes that a Sotah who does not drink does not get her Kesuvah because it is a Safek. However, he says (1:15) that if one of the women who hate her testified, she receives her Kesuvah. This requires investigation.