Gemara

(Mishnah): Within 30 days (of Kenisah, i.e. entering the Yavam's house) if she says that he did not do Bi'ah, we force him to do Chalitzah).

(R. Yochanan): Our Mishnah is like R. Meir, who says that a man can refrain (up to) 30 days:

(Beraisa - R. Meir): A Chasan has 30 days to claim that he found that his wife is not a virgin;

R. Yosi says, if they were secluded, he must make his claim immediately. If they were not secluded, he may make his claim even after several years.

(Rabah): The Mishnah is even like R. Yosi. R. Yosi said only that a man does not withhold himself (when secluded) with his Arusah, since he is familiar with her. A Yavam is shy (to have Bi'ah) with his Yevamah,

Question: Rather than force him to do Chalitzah, we should force him to do Yibum!

Answer (Rav): The case is, she has a Get from him.

Question (Beraisa): If within 30 days a Yevamah says that the Yavam has not had Bi'ah with her, no matter what he says we force him to do Chalitzah.

If she says this 30 days after falling to Yibum, we request that he do Chalitzah;

If she says that he did Yibum and he denies this, he gives her a Get;

If he says that he did Yibum and she denies this, she needs a Get and Chalitzah, even if he later denies that he did Bi'ah. (Even though she has no Get, within 30 days we force him to do Chalitzah!)

Answer #1 (R. Ami): Really, she already has a Get. The Seifa teaches that she needs Chalitzah in addition to her Get. (Even if he retracted, she needs Chalitzah, but we do not force him.)

Answer #2 (Rav Ashi): She already received a Get from Zikah (forbidding Yibum). She needs Chalitzah and another Get (because he said that he did Bi'ah, even though he later retracted).

A Yavam and Yevamah came before Rava; both said that they had not done Yibum (after he initially claimed that they did).

Rava: Do Chalitzah, and she is permitted.

Question (Rav Sharbiya - Beraisa): She needs a Get and Chalitzah.

Rava: If the Beraisa says so, that is the law.

Question (Hon): What is the law of the Tzarah (of a Yevamah who has been living with the Yavam and claims that she has not had Bi'ah)?

Answer (Rav Nachman): We will not forbid the Tzarah just because we force or request the Yavam to do Chalitzah!

Rishonim

Rif and Rosh (13:10): Rav says that we force him to do Chalitzah, not Yibum, because he gave to her a Get before Bi'ah and forbade her to him. A Beraisa teaches that within 30 days if she says that he did not do Bi'ah, no matter what he says we force him to do Chalitzah. After 30 days, we request. If she says that he did Yibum and he denies this, he gives her a Get. If he says that he did Yibum and she denies this, she needs a Get and Chalitzah, even if he later denies the Bi'ah. A case occurred and Rava initially ruled that Chalitzah suffices, until he heard that a Beraisa requires also a Get.

Nimukei Yosef (DH Masnisin): The Mishnah says that if he admits, even after 12 months we force him to do Chalitzah. Presumably this is even if he says that now he had Bi'ah. His admission that 30 days passed without Bi'ah is grounds to believe that even now he did not have Bi'ah.

Nimukei Yosef (DH Gemara): A man can refrain for 30 days, therefore she is believed within 30 days and we force him, so she will not be an Agunah. There is a slight Chazakah that a man does not refrain for 30 days, so after this we only request.

Question: Even after 30 days we should force him, for he does not lose through Chalitzah!

Answer #1 (Tosfos Reish 111b): He does not want to be embarrassed in Beis Din (she spits at him).

Answer #2 (Nimukei Yosef, ibid): Even though he agrees that he divorced her, he can say that he does not want to do Chalitzah, for perhaps he will want to remarry her.

Question: Since she said that he divorced her without Bi'ah, she said that he is permanently forbidden to her. She cannot remarry him!

Answer #1 (R. Akiva Eiger, Teshuvah 99 DH ha'Umnam): This supports the Ro'oh, who says that one does not become forbidden through his admission about an Isur mid'Rabanan (here, doing Yibum after a Get).

Answer #2 (Aruch l'Ner 111b DH b'Emtza): The Yavam hopes to persuade her to give an excuse for denying the Bi'ah, i.e. she hated him.

Note: According to the Nimukei Yosef we force a Kohen, for he does not lose. Perhaps we force even a Yisrael when she is permitted forbidden to him, e.g. through a vow that cannot be permitted, or if a Zar was Mekadesh her (he cannot be Machazir Gerushaso). It would be a Chidush to say that we do not force him because she forbade herself to him. He says that he permitted her to all men!

Question: According to Rav Ashi, why does she need another Get? She should be believed (that there was no Bi'ah and she needs no Get), Migo (since) she could have said that the Get was after Bi'ah!

Answer (Nimukei Yosef, ibid): Rashi explains that her Get explicitly says that it is for Zikah. Alternatively, a Migo is not believed against a Chazakah (that a man does not refrain). In monetary cases it is unresolved whether or not a Migo is believed against a Chazakah (Bava Basra 6a). Regarding Isur a Migo is not believed, especially here, for his claim supports the Chazakah.

Rif and Rosh (ibid.): We do not do anything (to forbid the Tzarah) just because we force or request the Yavam to do Chalitzah to the Kenusah.

Rosh (ibid.): Rashi explains that even if both deny that they had Bi'ah, since they were secluded together she is not believed to forbid the Tzarah. Her admission forbids only herself. The Yerushalmi supports this. It says that just like he cannot forbid the Tzarah, he cannot create a Safek about the father of her child (after Yibum). Rashi explains that the case when she says that he did Yibum and he denies this is after 30 days. She is believed because a man does not refrain for 30 days. Within 30 days we believe him to forbid her, even though she contradicts him. All the more so he forbids her Tzarah, who cannot contradict him!

Rambam (Hilchos Yibum 2:5): If within 30 days of Yibum (entering his house) a Yevamah says that he has not had Bi'ah with her, even though he says that he did Bi'ah and divorced her, we force him to do Chalitzah, since he divorced her.

Magid Mishneh: The Get is not before Kenisah, for if so she would be forbidden and he could refrain for 30 days.

Question (Lechem Mishneh): Rav Ashi requires a second Get for the Bi'ah. If a man can refrain when she is forbidden, she should be believed!

Rambam (ibid.): If he did not divorce her, we force him to do Yibum or do Chalitzah and divorce. If he divorced her after 30 days and she says that he did not do Bi'ah, we request that he do Chalitzah. If he admits that he has not had Bi'ah we force him to do Chalitzah. If she says that he did Yibum and he denies this, she does not need Chalitzah. He is not believed to forbid her after Kenisah.

Magid Mishneh: The Rambam holds that the latter clauses of the Beraisa apply before and after 30 days. Alternatively, the case in which both admit is even within 30 days, but unlike Rashi, he did not initially say that he did Bi'ah. There is a Chazakah that a man does Bi'ah immediately after Kenisah, therefore we always require a Get, lest people say that a Yevamah leaves with Chalitzah alone after Yibum. A proof of this is that we permit the Tzarah based on this Chazakah.

Question (Lechem Mishneh): Why must we mention the concern lest people say that she leaves with Chalitzah after Yibum? Since we permit the Tzarah, we are sure that there was Bi'ah!

Poskim

Shulchan Aruch (EH 167:6): If within 30 days of Yibum a Yevamah says that he has not done Bi'ah, even if he denies this, if he divorced her we force him to do Chalitzah. If he did not divorce her, we force him to do Yibum or do Chalitzah and divorce.

Rema: Even if he admits that he has not had Bi'ah she needs a Get and Chalitzah, for the Chazakah is that any Kenusah had Bi'ah.

Gra (8): There are four opinions. The Rambam and Tosfos' second opinion hold that the entire Beraisa applies before and after 30 days. Tosfos' first opinion holds that it is all after 30 days. Rashi holds that two clauses are after 30. Whenever both deny Bi'ah she needs only Chalitzah, and in every case the Tzarah is permitted. The Rosh holds like Tosfos' first opinion, except that the Tzarah is forbidden if within 30 days he denies having done Bi'ah.

Shulchan Aruch: If he divorced her after 30 days and she says that he did not do Bi'ah, we request that he do Chalitzah.

Gra (12): She says that it is a Get from Zikah; he says that the Get is from after Bi'ah. Even though both agree that Chalitzah suffices for her, we are concerned for both of their claims. Perhaps it is a Get from Zikah, and now she needs a Get from Bi'ah.

Shulchan Aruch (ibid.): If he admits that he has not had Bi'ah we force him to do Chalitzah. If she says that he did Yibum and he denies this, she does not need Chalitzah. He is not believed to forbid her after she entered his house.

Rema: Some say that he is believed within 30 days, and she needs Chalitzah with the Get.

Shulchan Aruch: In any case her Tzarah is permitted, even if they both say that there was no Bi'ah, even within 30 days.

Rema: Some say that he is believed within 30 days to forbid the Tzarah.

Beis Yosef (DH u'Mah she'Chosav Rabeinu dela'Da'as and Bedek ha'Bayis): According to one version, the Ri says that within 30 days the Tzarah always needs Chalitzah, even if both admit to Bi'ah, and a Kenusah always needs a Get, even if both deny Bi'ah. The Rosh asks why the Rif omits the law of the Tzarah. (Our text of the Rif brings it; the Rambam omits it.) It seems that he omits it because she is always permitted, and we have no reason to think otherwise. The Rif, Rambam and R. Chananel permit, and also the simple reading of Rashi permits, so we follow them, even though the Ri was unsure and the Rosh says that Rashi forbids.

Beis Shmuel (9): It seems that the text of the Beis Yosef should say that whenever both of them say that there was Bi'ah they are believed, even within 30 days, like Tosfos says. Tosfos forbids the Tzarah even after 30 days if both of them say that there was no Bi'ah.