Gemara

Question: "V'Ed Ein Bah" teaches that one witness suffices for Bi'ah, but not for warning or seclusion. What is the source to require two witnesses about Bi'ah if there was no warning and seclusion?

Answer: It says "Ki Matza Vah Ervas Davar", and "according to two witnesses... a Davar..." Just like monetary cases require two witnesses, also seclusion.

Kidushin 65b (Beraisa): Two men and a woman came from abroad with a package. Each man claims 'the other man is my slave. She is my wife. The package is mine. She claims 'they are both my slaves. The package is mine. She needs a Get from each man, and she collects a Kesuvah from the package.

Suggestion: Each man has one witness who says that he was Mekadesh her.

Objection: This cannot be. One witness who is contradicted is not believed!

66a - Question: If Reuven told Shimon 'your wife was Mezanah (forbidding her to you)' and Shimon was quiet, what is the law?

Answer #1 (Abaye): Reuven is believed.

Answer #2 (Rava): He is not believed. Matters of Ervah require two witnesses.

Support (Abaye, for himself): A case occurred in which a man told Levi 'I saw that your wife was Mezanah.'

Shmuel (to Levi): You must divorce her only if you believe him.

Shmuel meant, if you believe that he is not a thief (i.e. a wanton liar)!

Rejection (Rava): No. If you trust him like two witnesses, you must divorce her.

Kesuvos 9a (R. Elazar): A woman becomes forbidden through two witnesses. If there were (two witnesses about) warning and seclusion, even one witness (of Bi'ah) forbids her (permanently, and she may not drink the Sotah water).

Nedarim 90b (Mishnah): At first, if a woman told her husband 'I was defiled; I am forbidden to you', Beis Din would force him to divorce her and pay her Kesuvah:

The law was revised, lest women say this in order to marry someone else. If she says 'I was defiled', she is not believed without proof.

Rishonim

Rif and Rosh (Kidushin 28b and 3:15): The Halachah follows Rava, who says that Zenus is Devar sheb'Ervah; two witnesses are required. This is b'Dinei Adam. If one wants to fulfill Shamayim's will, if he believes the witness like two, he must divorce her. A woman becomes forbidden through two witnesses. If there were warning and seclusion, even one witness forbids her.

Rambam (Hilchos Ishus 24:17): If a man saw that his wife was Mezanah, or he was told by one of his or her male or female relatives whom he believes, he must divorce her and may not have Bi'ah with her. He must pay her Kesuvah.

Rashba (1:1237): One who believes one witness of Zenus must divorce her only if she is silent. One witness who is contradicted is not believed (Kidushin 65b), i.e. at all. It did not say ' he is not believed like two'! We must say that in the case with Shmuel, the man's wife was silent. Also Rashi (66a DH Afka) says 'if you believe him, divorce her, since she did not contradict him.' If it seems that a man falsely claims that his wife was Mezanah, even though he said that she is forbidden to him, he must fulfill his conjugal obligations to her. Beis Din uproots the Torah, so people will not do such things. We do similarly when a woman claims nowadays that she is forbidden to her husband.

Rambam (18): Beis Din forces a man to divorce his wife only if two witnesses say that she was Mezanah willingly in front of them.

Beis Meir (EH 115:7 DH Mi): The Re'em (Teshuvah 25) asks why the Rambam holds that we do not force him to divorce her when he himself saw. He admits that she is forbidden to him, just like a Chasan who says that she was not a virgin (Kesuvos 9a)! Mahari Levi (39) says that the Rambam holds that we force only when there are witnesses. Perhaps this is because we force only in a case when Beis Din punishes (for Bi'ah). Yevamos 24b supports this. (It says that Beis Din forces only through witnesses.)

Note: Nedarim 90b says that Beis Din used to force a man to divorce his wife if she said 'I was defiled.' The law changed only because she is not believed!

Hagahos Maimoniyos (10): Nowadays, R. Gershom's Cherem forbids divorcing one's wife b'Al Korchah (against her will). Therefore, also he is not believed to say that she was defiled, lest he seeks to marry someone else.

Poskim

Shulchan Aruch (EH 115:7): If a man saw that his wife was Mezanah, or he was told by one of his or her male or female relatives whom he believes, he must divorce her and may not have Bi'ah with her. He must pay her Kesuvah.

Rema: Some allow him to say that he believes the witness only if he relies on him also for other matters. If he believes him only about this because he already was suspicious about his wife, she is not forbidden to him.

Question (Beis Yosef DH v'Ha): Why did the Tur say that when he believes the witness, he divorces her merely to avoid Lazos Sefasayim (scandalous talk)? The Rif, Rambam and Rosh obligate divorcing her, and forbid Bi'ah. The Tur himself says so in Siman 178! Perhaps this is because here he added that she is silent. This requires investigation.

Answer #1 (Taz 11): Our text of the Tur is mistaken. The Maharshal says that in Siman 178 it should say (like the Gemara and Tosfos) 'if he is silent.' This is like admission. What is the case here? If he contradicts the witness, surely he need not divorce her! The Maharshal says that he asks 'how do you know this?' How does this help? He believes the witness like two! Lazos Sefasayim connotes that really, there is no Isur. If he believes him like two, there is real concern! Also, one need not divorce due to Lazos Sefasayim! Rather, the text should say 'b'Dinei Shamayim', not 'Lazos Sefasayim.' Beis Din cannot force him, for they do not know what is in his heart. Perhaps he retracted, and no longer believes the witness!

Answer #2 (Bach 115 DH v'Im): The Tur says that there is Lazos Sefasayim if he does not divorce her to fulfill Dinei Shamayim. The Maharshal says that if first the husband doubted the witness, and later he believed him, he must divorce her only due to Lazos Sefasayim. There is no source for this.

Shulchan Aruch (178:9): If one did not warn his wife, and one witness told him that she was Mezanah and she was silent, if he trusts the witness like two witnesses, he divorces her and pays her Kesuvah. If not, she is permitted to him.

Beis Shmuel (8): The Maharshal says the text must say if he is silent. If she were silent, he must divorce her even if he does not believe the witness!

Rema: The same applies if she herself says that she was Mezanah. Some say that nowadays that one cannot divorce b'Al Korchah, he cannot claim that he believes her or one witness. Perhaps he falsely claims to believe her, because he seeks to marry another woman. We excommunicate him for saying that he believes her, and thereby nullifying R. Gershom's Cherem.

Rema: Some say that even nowadays he is believed. If he had a quarrel with her he is not believed. Surely, he says that he believes the witness due to hatred!