Gemara

4a (Rav Sheshes): "Lo Sachsom Shor b'Disho" is followed by Parashas Yibum, to teach that we do not silence a woman who fell to Yibum to a Mukeh Shechin (he is covered with boils; she can refuse to do Yibum).

39b (Mishnah): Nowadays that people do not intend for the Mitzvah, Chalitzah takes precedence.

(Rami bar Chama): They returned to say that Yibum takes precedence, like Chachamim:

(Beraisa - Aba Sha'ul): If one did Yibum... for any reason (other than the Mitzvah), it is as if he transgressed Arayos. In my eyes, the child is nearly a Mamzer;

Chachamim say "Yevamah Yavo Aleha" - for any motivation.

112a - Question: If a married woman vowed not to have Bi'ah with any Yisrael (and fell to Yibum), what is the law?

Did it cross her mind that she might fall to Yibum (and she intended to forbid Yibum), or not?

Answer #1 (Rav): The Yavam is not like the husband (it did not cross her mind).

Answer #2 (Shmuel): The Yavam is like the husband.

Support (for Rav - Abaye - Mishnah): If a woman vowed in her husband's lifetime not to benefit from her Yavam, we force him to do Chalitzah.

If a woman thinks that she may fall to Yibum, we should only request that he do Chalitzah!

Rejection: The case is, she had children. She did not expect that her children will die and then her husband.

Inference: If she had no children, we only request that he do Chalitzah.

Objection (Seifa): If she intended to prevent Yibum, we request that he do Chalitzah.

The Seifa should distinguish without changing her intent, i.e. we force him to do Chalitzah only if she had children. If not, we only request!

Conclusion: Rather, whether or not she had children, we force him to do Chalitzah, like Rav.

Kesuvos 63a (Mishnah): If a woman is Moredes (refuses to have Bi'ah), we reduce her Kesuvah by seven Dinarim every week.

64a (Shmuel): We write an Igeres Mered (document of rebellion) against an Arusah (who refuses to go to Chupah), but not against a Yevamah (who refuses to do Yibum).

Question (Beraisa): We write an Igeres Mered even against a Yevamah.

Answer: The Beraisa is like the initial Mishnah. Shmuel is like the latter version:

(Mishnah): At first, when people intended for the Mitzvah, Yibum was preferable. Nowadays, Chalitzah is preferred.

(R. Pedas): We honor a demand to do Chalitzah, but not a demand to do Yibum.

Kesuvos 77a (Mishnah): If a husband has any of the following problems, we force him to divorce his wife: he is a Mukeh Shechin,... or a tanner, whether these preceded the marriage or came during the marriage;

R. Meir says, regarding all of them, even if he stipulated with her, she can say 'I thought I could bear it, but now I see that I cannot.'

Chachamim say, she is forced to bear it, except for a Mukeh Shechin, since this harms her.

A case occurred in Tzidon, and a tanner's wife fell to Yibum to a tanner. Chachamim said that she can claim, I could bear your brother, but not you.

Rishonim

Rif (Kesuvos 27b): Shmuel taught that we write an Igeres Mered against an Arusah, but not against a Yevamah. Shmuel is like the Mishnah Acharonah. Since Rav Asi bar Ada (our text - Rami bar Chama) taught that they returned to say that Yibum takes precedence, like Chachamim (Mishnah Rishonah), the Halachah does not follow Shmuel. Yeshivos (in the days of the Ge'onim) argued about whether or not we hold like Shmuel.

Ran (DH Kosvin): If he wants to do Yibum and she wants to do Chalitzah we do not force her, for we are happy with her rebellion, for (nowadays) people intend for beauty or money so Chalitzah has precedence.

Hasagos ha'Ra'avad: Presumably, we write an Igeres Mered only if he claims that he cannot remarry as long as she is Zakukak to him. We do not write if he can find another wife. Also, she is Moredes only if she refuses to do Chalitzah or Yibum. If he wants to do Yibum and she wants to do Chalitzah, she is not Moredes.

Question: If she refuses to do either, she should be considered Moredes even according to Mishnah Acharonah!

Answer: Since he wants to do Yibum, which is improper according to Mishnah Acharonah, we do not heed his claim.

Rif and Rosh (Yevamos 39a and 13:11): If a married woman vowed not to have Bi'ah with any Yisrael and fell to Yibum, we say that it did not cross her mind that she might fall to Yibum, like Rav. We force him to do Chalitzah; she is not considered Moredes. We infer that had she vowed after her husband died she would be considered Moredes. This shows that the Halachah does not follow Shmuel, who says that we do not write an Igeres Mered against a Yevamah.

Rebuttal (R. Efrayim, brought in Rosh and Ba'al ha'Ma'or): We cannot learn from here to Igeres Mered. Shmuel says that we do not force him to do Chalitzah, and that we do not write an Igeres Mered against a Yevamah! If this is inconsistent, the Gemara should have asked! Rather, forfeiting the Kesuvah does not depend on whether or not we force him to do Chalitzah.

Defense (Sefer ha'Zechus and Milchamos Hash-m): Rav and Shmuel discuss according to Mishnah Rishonah (Nedarim 90b, that if Reuven's wife vowed Hana'ah from all men, he divorces her and does not pay a Kesuvah). Shmuel said that we do not write an Igeres Mered against a Yevamah according to Mishnah Acharonah, which says that Reuven annuls the part of the vow that applies to him, and she is permanently forbidden to other men (until she goes to a Chacham to permit her vow). Chachamim are happy with her vow, for it avoids the problems of Yibum.

Rambam (Hilchos Yibum 1:2): If the Yavam or Yevamah does not want to do Yibum, he does Chalitzah and then she is permitted. The Mitzvah of Yibum takes precedence over the Mitzvah of Chalitzah.

Rambam (2:10): If a Yevamah permitted to do Yibum does not want to, she is like one who is Moredes against her husband. We force him to do Chalitzah; she does not receive her Kesuvah.

Magid Mishnah: The Rambam (Ishus 14:8) holds that we force any husband to divorce a Moredes. Whenever the Gemara says that we request, this refers to paying a Kesuvah. Some say that when he is unfitting we do not write Igeres Mered. This seems correct, for then Yibum is not preferable.

Rosh (Yevamos 4:17): The Gemara that says that we do not write an Igeres Mered against a Yevamah holds that Chalitzah has precedence.

Nimukei Yosef (ibid.): We expound only that we do not silence a Yevamah who fell to a Mukeh Shechin, i.e. we do not force her to do Yibum or write Igeres Mered against her. This is no source to force him to do Chalitzah. R. Shimshon imposed a Cherem not to marry a second wife. It does not apply here, for Shomayim gave her to him. However, we do not write Igeres Mered.

Poskim

Shulchan Aruch (EH 165:1): The Mitzvah of Yibum has precedence over Chalitzah. If she does not want to do Yibum with any brother (or with the oldest when he wants to do Yibum) without a proper reason, she is like a Moredes. Some say that Chalitzah has precedence.

Rema: According to the latter opinion she is not like a Moredes if she does not want to do Yibum.

Beis Yosef (159 DH Kosav ha'Rashba, citing the Rashba (802)): According to the Rif and Rambam, if a Shomeres Yavam vowed to forbid her Yavam after her husband died, she is considered a Moredes. The same applies if she became Mekudeshes to someone else. However, I hold like those who disagree. However, even if she claims that he is not fit for her we do not force him to do Chalitzah, for she is to blame.