Gemara

(Abaye): All agree that a Mamzer does not result from Bi'ah with a Nidah or a Sotah.

Support (Beraisa): All agree that the child of a Nidah, Sotah, or a Shomeres Yavam (l'Shuk, i.e. from a man other than her Yavam) is not a Mamzer.

Abaye did not teach the case of Shomeres Yavam, for he was unsure if the Halachah follows Rav or Shmuel.

92b (Rav): "Lo SiHYeH Eshes ha'Mes Chutzah l'Ish Zar" - HavaYaH (Kidushin) does not take effect on a Yevamah l'Shuk.

(Shmuel): Due to our ignorance, (if someone else was Mekadesh her) she needs a Get (to marry anyone else).

Shmuel is unsure whether "Lo Sihyeh" teaches that this is a Lav, or that Ein Kidushin Tofsin.

(Ameimar): The Halachah follows Shmuel.

92a (Mishnah): A woman's husband and their only son went overseas. They told her that her husband died and then her son died. She remarried. Later, they told her that her son died first (so she was Zekukah l'Yibum). She must leave her husband. Her children from him, both the first and last (before and after her husband truly died), are Mamzerim.

(Beraisa): This Mishnah is R. Akiva, who says that Ein Kidushin Tofsin b'Chayavei Lavin. Chachamim say that a Mamzer does not result from a Yevamah l'Shuk (she married anyone other than her Yavam).

92b (Rav Gidal citing Rav): Ein Kidushin Tofsin with a Yevamah. Nisu'in takes effect.

Question: If Kidushin does not take effect, neither does Nisu'in!

Answer #1: Rather, she has neither Kidushin nor Nisu'in.

Answer #2: 'She has Nisu'in' like Rav Hamnuna taught, that Bi'as Zenus forbids a Yevamah to her Yavam.

Answer #3: Nisu'in takes effect to require a Get. This is a decree lest she be confused with a woman who remarried based on one (false) witness.

Gitin 80b (Mishnah): If a man died without children (and Sarah remarried because she was Tzaras Ervah, and the Ervah was found to be an Ailonis, all the fines apply).

Version #1 - Suggestion: The fines apply when Sarah remarried, but if she was Mezanah, she is not fined. This refutes Rav Hamnuna, who says that if a Shomeres Yavam had relations with a stranger, she is forbidden to the Yavam.

Rejection: No, the fines apply whether she married or was Mezanah. The Tana discusses marriage, for this is nicer.

Version #2: Suggestion: The Mishnah discusses when Sarah remarried. The same applies if she was Mezanah. This supports Rav Hamnuna.

Rejection: No, the fines apply only when she got married, for then she might be confused with a woman who remarried after hearing (false testimony) that her husband died overseas.

Rishonim

Rif and Rosh (17a and 4:39): All agree that the child of a Shomeres Yavam (l'Shuk) is not a Mamzer.

Nimukei Yosef (DH v'Al): Even though there is a Lav, it says "Sihyeh" to teach that Kidushin Tofsin.

Ran (Gitin 42a DH Gemara): According to Chachamim there is never Mamzer mid'Oraisa from a Yevamah l'Shuk, therefore mid'Rabanan there is no Mamzer.

Rambam (Hilchos Ishus 4:14): If a Yevamah became Mekudeshes to a stranger she is Mekudeshes mi'Safek. Chachamim were unsure whether Kidushin Tofsin b'Yevamah, like other Chayavei Lavin, or if Ein Kidushin Tofsin, like Ervah. If he was Mekadesh, he divorces with a Get.

Question (Lechem Mishneh): The Rambam rules like Shmuel, who is unsure whether "Lo Sihyeh" is only a Lav, or also teaches that Ein Kidushin Tofsin. If so, why does he say (Yibum 2:19, brought below) that there are lashes for Bi'ah? Also, in Hilchos Isurei Bi'ah (15:1) the Rambam says that Mamzerim come only from Arayos. This connotes that the child of a Yevamah l'Shuk is not even a Safek Mamzer. Abaye considers him a Safek because he holds like Shmuel!

Answer (Lechem Mishneh): The Rambam holds like Tosfos (Yevamos 49b DH Iy), who says that Shmuel agrees that "Lo Sihyeh" is a Lav. His Safek is whether it also teaches that Ein Kidushin Tofsin. Tosfos (DH ha'Kol) says that even Rav holds that the child is not a Mamzer. He resembles the son of a Nochri or slave from a Bas Yisrael, who is not a Mamzer because the father cannot be Mekadesh anyone.

Rambam (3:19): A woman's husband and their only son went overseas. They told her that her husband died and then her son died. She remarried. Later, they told her that her son died first (so she was Zekukah l'Yibum). She must leave her husband. Her child from him is Kosher.

Yam Shel Shlomo (10:23): The last two versions of Rav Gidal are like the Sugya in Gitin, therefore the Halachah follows them. We are stringent about a Yevamah who remarried lest she be confused with an Eshes Ish, therefore all 13 fines apply, including that her children are Mamzerim.

Question: Why does the Gemara (Yevamos 92a) say that only R. Akiva says that children of a Yevamah l'Shuk are Mamzerim? We find in Gitin (80b) that Chachamim agree that they are Mamzerim! The Mishnah said that if a Yevamah married l'Shuk without Chalitzah because she thought that she was exempt (and later found that she was Zekukah), her children are Mamzerim (and many other fines apply). The Gemara deliberated whether this supports or refutes Rav Hamnuna, who says that if a Shomeres Yavam was Mezanah is forbidden to her Yavam. Surely the question is according to Chachamim. In Sotah (18b), a Mishnah teaches like Rav Hamnuna, but the Gemara says that it does not support him, for it is like R. Akiva.

Answer (Rosh 10:3): Chachamim say that the child is a Mamzer mid'Rabanan. Our Mishnah connotes that he is a Mamzer mid'Oraisa, since the Seifa discusses a woman who remarried (based on false testimony) before her husband truly died. It says that her children from him after he died are not Mamzerim. Surely they are Mamzerim mid'Rabanan; the Mishnah discusses Mamzerus mid'Oraisa. If you will say that the Mishnah discusses when two witnesses had testified and therefore she is not fined, we should say the same regarding the Yevamah! Some say that only when the husband returns and we fine to make her future children from him Mamzerim, we also make future children from her new husband Mamzerim (even after her first husband divorced her). This is wrong. The primary fine is against the latter husband, who married her b'Isur! The Gemara (Yevamos 15b) says that Kohanim Gedolim descended from Tzaros Ervah who married without Chalitzah, even though Beis Shamai require Chalitzah. This is not difficult, for Chachamim decreed to disqualify only when it looks like a woman whose husband returned after she remarried based on one witness. The Gemara (94a) says that according to Chachamim, there is no Kilkul if a Yevamah married without Chalitzah. We must say that that Gemara is unlike the Sugya in Gitin, rather, like the first version of Rav Gidal (Nisu'in does not apply to a Yevamah) and the Yerushalmi. Alternatively, since it is more common that the Yevamah hates the Yavam than loves him, it is reasonable that we are concerned only for Kilkul mid'Oraisa, even though normally we are concerned even for Kilkul mid'Rabanan. The Gemara there concludes that we are not concerned even for Kilkul mid'Oraisa, therefore it did not need to say that we are not concerned for Kilkul mid'Rabanan. The Yerushalmi says that the Mishnah (of a Yevamah who married l'Shuk (based on false testimony) is like R. Akiva, but Chachamim say that Mamzerim do not result from a Yevamah.

Rosh (ibid.): Even Chachamim agree that the son of a Yevamah l'Shuk is a Mamzer mid'Rabanan, because the case is confused with a woman whose husband returned after she remarried based on one witness. Some say that the Sugya in Gitin forbids because she resembles a woman who remarried based on one witness, but the child is not a Mamzer, like the Yerushalmi says. This is wrong; the Gemara says that all the fines apply. However, one can say like the Rif and R. Chananel that the Halachah follows the Yerushalmi. The son of a Yevamah l'Shuk is not a Mamzer, but all other fines apply lest she be confused with an Eshes Ish.

Poskim

Shulchan Aruch (EH 7:14): If a Kohen had relations with a Yevamah l'Shuk, the child is not a Chalal only if she conceived from the first Bi'ah. After the first Bi'ah, she is a Zonah.

Chelkas Mechokek (25): The child is a Mamzer only if there was Nisu'in.

Shulchan Aruch (159:2): If she had Nisu'in with a stranger before Chalitzah, even if she was Shogeg he must divorce her, even if they had children. She becomes forbidden to him and to her Yavam, and the 13 fines (Siman 17:57) apply. However, the child is not a Mamzer. Some say that he is a Mamzer mid'Rabanan. She does Chalitzah and then she may marry someone else.

Beis Yosef (DH v'Hachi and v'Chen Kosav): The Stam Mishnah says that the child is a Mamzer (when she married due to a false report that her husband died before his son), but the Gemara says that this is like R. Akiva. The Rambam infers that according to Chachamim the child is not a Mamzer even mid'Rabanan. Tosfos and the Rosh say that the child is a Mamzer mid'Rabanan, because the case is confused with a woman whose husband returned after she remarried based on one witness.

Gra (14): Tosfos (92a DH Aval) says that the Mishnah (92a) cannot be like Chachamim and the children would be Mamzerim mid'Rabanan, for the Seifa says that if a woman remarried based on one witness and her husband returned, later children are not Mamzerim. Surely they are Mamzerim mid'Rabanan! We must say that the Mishnah discusses Mamzerus mid'Oraisa. The Ramban disagrees, for even from the first husband only children conceived before the latter divorces her are Mamzerim; children of a Sotah are not Mamzerim (49b).

Chazon Ish (20:6): The Shulchan Aruch says even if she was Shogeg. Tosfos and the Nimukei Yosef say that b'Mezid is not considered Nisu'in. It is mere Zenus. The fines do not apply to an Eshes Ish who married b'Mezid, and Yevamah is fined only due to the similarity to Eshes Ish! However, this is according to Rav. The Halachah follows Shmuel, who says that Safek Kidushin Tofsin b'Yevamah, so we must be stringent even if she was Mezid.