Gemara

20a (Mishnah): A widow to a Kohen Gadol (does Chalitzah, not Yibum).

Question: Granted, if she was widowed from Nisu'in, a Lav and an Aseh forbid her, so an Aseh (Yibum) does not override them. But if she was widowed from Kidushin, only a Lav forbids her. The Aseh should override the Lav!

Answer #1 (Rav Gidal): "Yevimto will ascend to the gate" is extra, to teach that there are Yevamos who do Chalitzah but not Yibum, i.e. Chayavei Lavin.

Objection (Rava - Beraisa): A Petzu'a Daka does Chalitzah or Yibum. If he did Yibum, he acquires her, but they may not remain married - "Lo Yavo Fatzu'a Daka...(bi'Khal Hash-m)".

If mid'Oraisa, Chayavei Lavin do Chalitzah but not Yibum, why does he acquire her through Yibum?

Answer #2 (Rava): Also a widow from Kidushin is forbidden to a Kohen Gadol by an Aseh (as well as a Lav), "Kedoshim Yihyu."

Question: How can we answer for a Mamzeres or Nesinah? We cannot say that "V'Hiskadashtem" is an Aseh (even for Yisraelim), for if so, every Lav in the Torah is also an Aseh!

Answer #3 (Rava): Chachamim decreed that a widow from Kidushin may not do Yibum, lest a widow from Nisu'in do Yibum.

Question: How can we answer for Mamzeres or Nesinah? We cannot say that even though they are uncommon, Chachamim forbade them to do Yibum, lest one marry them without the Mitzvah of Yibum. If so, no woman should do Yibum, lest an Ailonis do Yibum!

Answer #4 (Rava): We forbid Yibum lest he have a second Bi'ah with her.

Support (Beraisa): If they did Yibum he acquires her with the first Bi'ah. A second Bi'ah is forbidden.

Retraction (and Answer #5 - Rava): My answer is wrong. Reish Lakish taught that an Aseh overrides a Lav only if one cannot fulfill both. Here, both can be fulfilled through Chalitzah!

Refutation (of Answer #5 - Beraisa): If they did Yibum, he acquires her.

(R. Yochanan or R. Elazar): If a Kohen Gadol did Yibum with a widow, the Tzarah is exempted.

(The other of R. Yochanan and R. Elazar): She is not exempted.

If she was widowed from Nisu'in all agree that the Tzarah is not exempt, for an Aseh does not override an Aseh and a Lav. They argue about a widow from Eirusin.

One opinion says that she exempts the Tzarah, because an Aseh overrides a Lav. The other opinion says that the Aseh doesn't override the Lav, since Chalitzah could be done.

Refutation (of the latter opinion - Beraisa): If they did Yibum, he acquires her.

Chulin 141a (Rava): One might have thought that one may take the mother for a Mitzvah (e.g. Taharas Metzora). "Teshalach" obligates sending it in any case.

Question: Why would we think that one Aseh is stronger than and overrides another?

Rishonim

Rashi (20b DH O Meyabmim): The Beraisa says that a Petzu'a Daka does Chalitzah or Yibum. Yibum is Asur, but if done it acquires.

Hagahos ha'Gra [1]: The text should say 'he does Chalitzah, not Yibum.'

Rif and Rosh (2:1): Isurei Mitzvah or Kedushah (Shniyos or Chayavei Lavin) do Chalitzah but not Yibum. This includes a widow to a Kohen Gadol, whether she was widowed from Kidushin or Nisu'in.

Rif (4b): Even a Mamzer creates Zikah to do Yibum. This teaches that mid'Oraisa, Chayavei Lavin can do Yibum, because Aseh Docheh Lo Sa'aseh if both cannot be fulfilled. Chachamim decreed against Yibum, lest he have a second Bi'ah. This is why if a Petzu'a Daka did Yibum, he acquires her, but he may not keep her for a second Bi'ah - "Lo Yavo Fatzu'a Daka..."

Rambam (Hilchos Yibum 6:10): If the Yevamah was forbidden to the Yavam by a Lav or an Aseh or she was a Shniyah, she does Chalitzah, not Yibum. Because Kidushin takes effect on them, they are Zekukos to Yibum. The Torah permits Yibum, for Aseh Docheh Lo Sa'aseh. Chachamim decreed to forbid Yibum of Chayavei Lavin and Shniyos, lest they have a second Bi'ah. The Mitzvah is only the first Bi'ah.

Magid Mishneh: Rav Gidal's Drashah (there is a Yevamah who does Chalitzah but not Yibum) applies to a widow from Nisu'in to a Kohen Gadol.

Tosfos (8b DH Melamed): We do not need a verse to permit a second Bi'ah in a normal Yibum. It is unreasonable that the Mitzvah is to divorce her afterwards. However, later Bi'os are only Reshus, therefore it is forbidden if she is Chayavei Lavin.

Tosfos (20b DH Atu): Really, the decree is lest he complete the Bi'ah. He acquires her through Ha'ara'ah (the initial stage of relations). After this is not a Mitzvah.

Rebuttal (Ramban Kidushin 10a DH v'Yesh): When the Torah discusses Bi'ah, e.g. "Yevamah Yavo Aleha", it refers to a full Bi'ah. Even though a Drashah teaches that he acquires through Ha'ara'ah, the entire Bi'ah is a Mitzvah and is permitted. Also, Yibum is "Lehakim Shem", which requires a full Bi'ah.

Maharatz Chayos (on Tosfos): Even Tosfos is not Mechayev for a full Bi'ah. This is like one who was initially forced to have Bi'ah and later desired it. She is exempt, for 'Yitzra Albesha' (the Yetzer ha'Ra overpowers).

Question: Since only one Bi'ah is permitted for Chayavei Lavin, a Besulah cannot become pregnant. The Yibum is not "Lehakim Shem"!

Answer #1 (Tosfos 20a DH Yavo): The Bi'ah need not be able to be Mekim Shem, just the people must be. This excludes a Seris and Ailonis.

Answer #2 (Korban Nesan'el (on Rosh 2:3) 1): A Besulah can become pregnant from the first Bi'ah if the Yavam is skilled, like Shmuel, just this is not common.

Note: This is like the Rishonim who allow a full Bi'ah.

Rambam (6:11): If a Kohen Gadol did Yibum with a widow from Nisu'in, the Aseh is not Docheh the Lav and Aseh, so he did not totally acquire her mid'Oraisa. Therefore, the Tzarah is not exempt needs Chalitzah.

Tosfos (20b DH Gezeirah): "Yevimto" teaches that a widow from Nisu'in or Chayavei Aseh (without a Lav) do Chalitzah but not Yibum.

Poskim

Shulchan Aruch (EH 174:1): If a Yevamah (Leah) was forbidden to the Yavam by a Lav or an Aseh or Shniyos mid'Rabanan, she does Chalitzah, not Yibum. Her Tzarah (Chanah) may do Chalitzah or Yibum.

Gra (1): Also when she is forbidden by an Aseh, the Isur Yibum is a decree lest he have a second Bi'ah.

Shulchan Aruch (ibid.): If the Yavam transgressed and had Bi'ah with Leah, he acquired her and the Tzarah is exempt.

Question: An Aseh does not override an Aseh, for it is no stronger (Chulin 141a). It should not acquire her totally, like when she is forbidden by a Lav and an Aseh! The Rambam does not mention a decree against Yibum of Chayavei Aseh, presumably because it is forbidden mid'Oraisa. But the Shulchan Aruch and Rambam say that Yibum with Chayavei Aseh acquires her totally!

Answer #1 (Beis Meir): Tosfos (20b DH Gezeirah) holds that mid'Oraisa she does not fall to Yibum, because one Aseh is not stronger than another. This is why Yibum of Chayavei Aseh does not even exempt the Yevamah, and she needs a Get and Chalitzah (Yam Shel Shlomo 2:4). The Pnei Yehoshua (Kesuvos 40a DH b'Tosfos) says that the only case of an Aseh, i.e. a first or second generation converted Mitzri or Edomi, is not a proper Aseh. It is only a Lav derived from an Aseh, so it is no stronger than a Lav. Only an Aseh fulfilled through action is stronger than a Lav. The Mishneh l'Melech (Na'arah Besulah 1:5, just before DH Gam) says so in the name of the Rashba.

Question (Beis Meir): If so, also the Aseh of a Nesu'ah to a Kohen Gadol is no stronger than a Lav. Why do we say that Yibum does not override the Lav and Aseh of a widow from Nisu'in to a Kohen Gadol? An Aseh overrides even more than one Lav, e.g. "Lo Sikach" and "Lo Yechalel" (when a widow from Kidushin falls to a Kohen Gadol)! The Rashba (20b DH uv'Hai) says that Yibum does not override a the Lav and an Aseh, or even a lone Aseh. This is unlike what the Mishneh l'Melech said in his name!

Note: To answer this, the Beis Meir says that the 'lone Aseh' the Rashba discusses is a proper Aseh, i.e. the Isur for a Sotah to marry the Bo'el. It is not clear how one fulfills this 'Aseh' through an action.

Answer #2 (Sha'agas Aryeh 33): A Lav is more severe than an Aseh, nevertheless an Aseh is Docheh a Lav. A passive Aseh is essentially a Lav.

Answer #3 (Teshuvas Noda bi'Yehudah (volume 1, Sof CM) DH Divrei): The Gemara never brings a source that an Aseh is not Docheh an Aseh. It is mere reasoning that one is not stronger than the other. It is better to transgress (one of them) passively than to transgress through an action. However, if he did an act to fulfill one of them, he was Yotzei. Yibum of Chayavei Lavin v'Aseh is different, for a Drashah teaches that there is a Yevamah who does Chalitzah but not Yibum.