Gemara

(Mishnah - R. Gamliel and R. Eliezer): If we saw a woman talking with a man and she says that it was Ploni, a Kohen, she is believed;

R. Yehoshua says, we do not rely on her. The Chazakah is that she had Bi'ah with a Nasin or Mamzer (and she is forbidden to Kehunah) unless she brings proof.

(Ze'iri): 'Talking' means that she was secluded with him.

(Rav Asi): She had Bi'ah with him. 'Talking' is a euphemism.

13b: An Arus and his Arusah came in front of Rav Yosef. Both said that she is pregnant from him.

(Rav Yosef): There is no reason to be concerned. Firstly, the husband admits. Also, Shmuel taught that the Halachah follows R. Gamliel!

Question (Abaye): Your connote that the second reason suffices by itself. Does R. Gamliel believe her when the husband does not admit?

Shmuel (to Rav Yehudah): Even though the Halachah follows R. Gamliel (regarding a single girl), be lenient only if most people in the area do not disqualify her. Here, everyone except for her husband disqualifies her!

Answer (Rav Yosef): If the Halachah follows R. Gamliel, why did Shmuel require a Kosher majority? You must say that the Halachah follows R. Gamliel b'Di'eved. L'Chatchilah, we require a Kosher majority. Also our case is like b'Di'eved!

Rishonim

The Rif brings our Gemara, and concludes 'here is b'Di'eved because they are an Arus and his Arusah.'

Question (Ran DH ha'Hu): Why is this called like b'Di'eved? Regarding the child it is l'Chatchilah. Regarding her it is truly b'Di'eved!

Answer (Ran): If we will call the child a Safek Mamzer, the Arus will worry (that she is forbidden to him) and will separate from her. Therefore it is called b'Di'eved even regarding the child. Alternatively, it is b'Di'eved regarding the child, for if we disqualify him he cannot marry a Yisrael. It is l'Chatchilah for her, for she could marry a Yisrael. The Ra'avad says that it is like b'Di'eved regarding her because there was no Chupah yet. It is not like l'Chatchilah because he was already Mekadesh her.

Rambam (Hilchos Isurei Bi'ah 15:12): If a single girl became pregnant and said that it is from a Mamzer, even if the Mamzer agreed the child is a Safek Mamzer. We say that she was Mezanah also with others.

Magid Mishneh: Mid'Oraisa, a Safek Mamzer may marry Kosher Yisraelim but not Mamzerim. Therefore the mother is believed to be Machshir her child, but not to disqualify.

Shev Shematsa (2:17): Why is she believed to Machshir herself? This is a Safek mid'Oraisa, about which we are stringent! Even the Rambam agrees that the Torah is stringent about a Safek mid'Oraisa when the Isur is Kavu'a. The Pnei Yehoshua says that we rely on the Chazakah that a woman checks before being Mezanah (that the Bo'el is Kosher). This is only to be Machshir, but not to disqualify. I say that even after this Chazakah it is like an even Safek (when the majority of men are Pesulim). Also, perhaps she was raped! We must say that R. Gamliel believed a Vadai claim when there is an even Safek. The same applies when she says that the child is Pasul and most men are Kesherim! Rather, a Vadai claim helps only for the one who claims (herself), but not for someone else (her child).

Rambam (17): If an Arusah became pregnant in her father's house, the Chazakah is that the child is a Mamzer. We do not establish her to be a Zonah; she is believed to say that she had Bi'ah only with the Arus. If she (was widowed and) married a Kohen she need not leave, and the child is Kosher.

Magid Mishneh: The Rambam holds that he is not believed to disqualify her. In any case she is forbidden to him, for he says that she had Bi'ah with someone else, and the only Safek is whether or not she was forced.

Rosh (1:29): We do not Machshir if the Arus does not agree. R. Gamliel is not Machshir when both of them are Vadai (16a).

Hafla'ah (13a DH b'Masnisin): (Amora'im argue about whether 'talking' in the Mishnah refers to seclusion or Bi'ah.) Rashi says that 'Kohen' merely means one with good lineage. It seems that it discusses specifically a Kohen, and he wants to marry her. R. Yehoshua holds that he is not believed to say that it was him, just like a Kohen who testifies that a Shevuyah (a girl taken captive) was not defiled may not marry her. Rav Yosef (14a) connotes that when the Arus admits, even R. Yehoshua permits. That refers to the Isur of a Safek Sotah to her husband. Regarding Kehunah, we learn from Shevuyah. A husband is not believed to say that his wife was not defiled (Yevamos 88b).

Poskim

Shulchan Aruch (4:27): If an Arusah became pregnant in her father's house and she says that she is pregnant from the Arus and the Arus agrees or is not here, the child is Kosher.

Bach (DH v'Im): A Chacham explains the Tur to say that when the Arus agrees or is not here the child is Kosher, and all the more so she is Kosher to Kehunah (after the Arus dies), for it is b'Di'eved for her. It seems that she is permitted to a Kohen l'Chatchilah only if the Arus agreed. If he was not here, l'Chatchilah she is forbidden to Kehunah because most men disqualify her. The Gemara in Kesuvos connotes that the Pesak like R. Gamliel permits (b'Di'eved) as much as the Arus' admission does. This is only regarding the child and her Heter to the Arus, but not to permit her to a Kohen.

Rebuttal (Chelkas Mechokek 28): The child may marry a Yisraelis and inherits with his brothers. If he is a Kohen the child may do Avodah, and she is permitted to her Arus. Therefore, (in every case) also regarding his mother it is like b'Di'eved and she is permitted to Kehunah after her husband dies. Once she was permitted (to him), she is permitted (to Kehunah). The Halachah follows R. Gamliel. It is a mere stringency to require two majorities, especially if she was not suspected of others.

Shulchan Aruch (ibid.): We do not establish her to be a Zonah. She is believed to say that she had Bi'ah only with the Arus, even if he contradicts her. This refers to marrying others. If she married a Kohen she need not leave, and the child is Kosher. She is forbidden to the Arus, for he says that she is forbidden to him.

Gra (67): The Magid Mishneh says that the Rambam says that we do not consider her to be a Zonah even when he contradicts her. It is unlike the Rosh. Alternatively, the Rosh agrees. He said that R. Gamliel is not Machshir when both are Vadai only regarding the child, who has no Chazakah of Kashrus. The Rambam explains that Rav Yosef was Machshir her even when the Arus does not agree, i.e. even if he disagrees. This is unlike the Rosh and Magid Mishneh. The Rambam agrees that the Arus is believed to disqualify the son.

Beis Shmuel (47): Surely l'Chatchilah she is forbidden to Kehunah, for her first son is a Mamzer. Nevertheless, if she married a Kohen she need not leave. Even though I said (44) that if he is his son for lineage, he is his son in every respect, this does not determine her status.

Yam Shel Shlomo (Yevamos 7:11 and Kesuvos 1:44): Even though Shmuel said not to permit l'Chatchilah without a second majority, since she is married to him it is considered b'Di'eved. If he died before Nisu'in, l'Chatchilah she may not marry a Kohen. If he died after Nisu'in, since we ruled that (she is not a Zonah and) he may marry her, not she may marry a Kohen l'Chatchilah. The Rambam and Tur say that he is not believed to make her a Zonah. This is wrong. R. Gamliel believes her only when she is Vadai and he is not! If he is more lenient about an Arusah, why did Abaye ask? Rav Yosef permitted an Arusah, and R. Gamliel discussed a single girl!