Gemara

(Mishnah) She answers "Amen" twice to accept the curse, and also the oath... that she was not Mezanah when she was a Shomeres Yavam, or after Yibum... The rule is, he cannot stipulate about any Bi'ah that would not forbid her to him.

(Gemara - Rav Hamnuna): If a Shomeres Yavam had Bi'ah with a stranger, she is forbidden to the Yavam.

He learns this from our Mishnah. The oath includes Zenus while she was a Shomeres Yavam and after Yibum. We understand this only if Bi'ah with a stranger forbids a Shomeres Yavam to the Yavam. The Mishnah says that he cannot stipulate about any Bi'ah that would not forbid her to him!

(Chachamim of Eretz Yisrael): The Halachah does not follow Rav Hamnuna.

He learned from our Mishnah, which (is unlike the Halachah. It is) like R. Akiva, who says that Kidushin is not Tofes (does not take effect) if a Lav forbids them to have Bi'ah. Therefore, a Yevamah who was Mezanah with a stranger is like a married woman who was Mezanah; she becomes forbidden to her husband.

Yevamos 79b (Mishnah): Chalitzah of a Seris (one who does develop like a normal male) does not disqualify a Yevamah (from Kehunah). If he had Bi'ah with her, he disqualified her, for it is Bi'as Zenus.

81a - Question (against Rav Hamnuna): Bi'ah of a Yavam Seris disqualifies her (she is Eshes Achiv; it is not a Mitzvas Yibum), but Bi'ah of a stranger does not!

Rejection: No, also Bi'ah of a stranger disqualifies her. Since the Reisha discusses the Yavam, also the Seifa does.

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

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

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

(Ameimar): The Halachah follows Shmuel.

(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!

Answers: Rather, she has neither Kidushin nor Nisu'in. Alternatively, 'she has Nisu'in' like Rav Hamnuna taught, that Bi'as Zenus forbids her to the Yavam. Alternatively, 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.

Rishonim

Rambam (Hilchos Yibum 2:20): If a Shomeres Yavam was Mezanah she does not become forbidden to the Yavam. He may do Chalitzah or Yibum.

Rambam (Hilchos Isurei Bi'ah 18:2,3): If a woman had Bi'ah with a man forbidden by a Lav that applies to everyone, she becomes a Zonah. This includes a Yevamah who had Bi'ah with a stranger.

Rosh (Yevamos 10:3): The first answer says that Rav Gidal taught that Nisu'in does not apply to a Yevamah. This is like the Yerushalmi, which says that the Mishnah (of a Yevamah who married l'Shuk) is like R. Akiva, but Chachamim say that Mamzerim do not result from a Yevamah. This is unlike Rav Hamnuna. Since Nisu'in b'Mezid does not forbid her, we do not fine b'Shogeg.

Nimukei Yosef (Yevamos 29a DH Gemara): The Halachah does not follow Rav Hamnuna. Zenus is not common, so Chachamim did not decree about it.

Yam Shel Shlomo (Yevamos 10:23): There is no basis in the Gemara whether or not the Halachah follows Rav Hamnuna. In any case we are lenient only about the Yavam, but she is forbidden to the Bo'el.

Shiltei ha'Giborim (29b 2): Ri'az says that most Poskim say that Zenus does not forbid her to the Yavam. This seems incorrect. She is like Safek Ervah, therefore she is forbidden.

Tosfos (18b DH Amrei): Even though 'they said in Eretz Yisrael' (which refers to R. Yirmeyah) that the Halachah does not follow Rav Hamnuna, it seems that the Halachah follows him. In various places (Yevamos 92b, 95a, Gitin 80b) the Gemara establishes Tana'im or Amora'im like Rav Hamnuna (and does not say that the Halachah does not follow him).

Poskim

Shulchan Aruch (EH 6:8): If a stranger had Bi'ah with a Yevamah, she is a Zonah.

Source (Gra 28): Beis Shamai say that Tzaras Ervah falls to Yibum, and her child from a stranger is Pasul to Kehunah (Yevamos 14b). The child of Machazir Gerushaso is Kosher, because she is not disqualified. This implies that a Yevamah l'Shuk is disqualified. However, Kohanim Gedolim came from Tzaros Ervah l'Shuk! We must say that this was from the first Bi'ah (before she became a Zonah), or that the Kohanim descended (maternally) from Tzaros who married Yisraelim without doing Chalitzah.

Question (Beis Yosef DH u'Mah she'Chosav Chutz): The Rosh says that a Zonah results from a Bi'ah in which he could not be Mekadesh her. However, since the Halachah follows Shmuel, we should say that she is a Safek Zonah!

Answer #1 (Taz 10): Indeed, the Rosh means that we are stringent to consider her a (Safek) Zonah.

Answer (Beis Shmuel 21): Really, Ein Kidushin Tofsin b'Yevamah. It is a mere stringency to require a Get.

Shulchan Aruch (159:3): Bi'as Zenus does not forbid a Yevamah to her Yavam.

Beis Yosef (DH Zinsah): R. Yerucham forbids if her husband is a Kohen.

Rema: Some say that there is a fine, and she is forbidden to the Bo'el.

Source #1 (Nimukei Yosef Yevamos Sof 29a): If a Yevamah was Mezanah with a stranger, she is permitted to her Yavam, for he did not sin. The Ritva says that we fine her, and forbid her to the stranger (even after Chalitzah).

Source #2 (Gra 17): The Terumas ha'Deshen forbids the Mekadesh to marry her. We do not hold like the second answer (Yevamos 92b) that forbids due to Zenus but not for Kidushin, but in any case Zenus is not more lenient!

Question (Beis Shmuel 9): Bas Sheva was permitted to David (even though she was married at the time) because she was forced. Since she was permitted to her husband, she was permitted to the Bo'el. (How can we be more stringent about a Yevamah?) This supports what I wrote (11:3) that the primary opinion is that she was permitted because she had received a Get.