Gemara

Chachamim expound "V'Yibmah (to her)" to teach that he may do Yibum against her will;

Rebbi learns this from "Yevamah Yavo Aleha". (Malbim - elsewhere the Torah discusses Bi'ah El Ishah, denoting equality. Here it says Aleha, to show that he is dominant.)

53b (Mishnah): If a Yavam had Bi'ah with his Yevamah he acquired her, whether he was Shogeg or Mezid (intending), whether forced or willing, even if he was Shogeg and she was Mezidah, or vice-versa, or he was forced and she was not, or vice-versa;

(Gemara) Question: Why does it say 'even (if he was Shogeg)'?

Answer: He acquires her not only when he is Shogeg or Mezid (intending for Zenus) and she intends for the Mitzvah. Rather, even when he is Shogeg and she is Mezidah, and neither intends for the Mitzvah, he acquires.

(Beraisa - R. Chiya): Even if both are Shogegim, Mezidim, or forced (he acquires her).

Question: In the Mishnah, what is the case of being forced?

Suggestion: Nochrim forced him to have Bi'ah.

Rejection: Rava taught that a man is never considered forced regarding Arayos, since Kishuy (an erection) comes only willfully!

Answer #1: Rather, the case is, he was sleeping.

Objection: Rav Yehudah taught that if he was sleeping (when he did Yibum) he did not acquire her!

Answer #2: Rather, he (had Kishuy intending for his wife and) fell and had Bi'ah with his Yevamah.

Objection: Rabah taught that if one fell from the roof onto a person, he pays four of the five payments of damage. If he landed on his Yevamah and had Bi'ah, he does not acquire her!

Answer #3: Rather, he intended (had Kishuy) for his wife, and his Yevamah grabbed his Ever and inserted it.

Question: R. Chiya discusses when they are both forced. Where do we find this?

Answer: He was intending for his wife, and Nochrim grabbed him and inserted his Ever into his Yevamah.

Question: What is the source that such Bi'ah acquires a Yevamah?

Answer (Beraisa #1): "Yevamah Yavo Aleha" is the Mitzvah (it is preferred to Chalitzah). "Yevamah Yavo Aleha", whether he is Shogeg or Mezid, whether forced or willing.

Question: We already expounded this to teach that that Yibum is the Mitzvah!

Answer: That is learned from "If he does not want". This implies that, if he wants, he should do Yibum. Our verse teaches that he acquires whether he is Shogeg or Mezid, whether forced or willing.

(Rav Yehudah): One who did Yibum while sleeping did not acquire his Yevamah. "Yevamah Yavo Aleha" teaches that he must intend for her.

Question (Beraisa): Whether he is awake or asleep (he acquires her).

Answer: Rather, it should say 'whether she is awake or sleeping.'

Question (Beraisa): Whether he is awake or asleep; whether she is awake or asleep (he acquires).

Answer: The case is, he is dozing.

Question: What is the case of dozing?

Answer (Rav Ashi): He is asleep but not (fully) asleep, and awake but not awake. If one calls to him, he responds, but cannot give an answer that requires reasoning. When one reminds him (what he heard), he remembers.

(Rabah): If one fell from the roof and had Bi'ah with his Yevamah, he did not acquire her.

(Rava): If one intended to stick his Ever in the wall and it landed in his Yevamah, he does not acquire her;

If he intended to stick it in an animal and it landed in his Yevamah, he acquires her, since he intended for a type of Bi'ah.

Rishonim

The Rif and Rosh (18a and 6:1) bring our Mishnah.

Nimukei Yosef (DH ha'Ba): Even mid'Rabanan he acquires even when he was Mezid, i.e. intending for Zenus. However, this is forbidden mid'Rabanan. This is why we did not say that our Mishnah is unlike Aba Sha'ul, who says 'Yibum for the sake of her beauty or any reason (other than the Mitzvah) is like transgressing Arayos. In my eyes, the child is nearly a Mamzer.' Chachamim say "Yevamah Yavo Aleha" (permits any intent). All forbid mid'Rabanan. They argue about whether or not it is like Ervah.

Beis Shmu'el (EH 166:5): The Nimukei Yosef says that Aba Sha'ul agree that b'Di'eved he acquired. Perhaps this is because the Nimukei Yosef holds that Aba Sha'ul forbids only mid'Rabanan. However, we can say that he acquires her even if the Isur is mid'Oraisa.

Rambam (Hilchos Yibum 2:3): If a Yavam had Bi'ah with his Yevamah he acquired her, whether it was Shogeg, Mezid, forced or willing. This is whether he was Mezid and she was Shogeges or forced, or if she was Mezidah and he was Shogeg or forced, and whether she was asleep or awake.

Rambam (4): This is if he intended for Bi'ah. If he fell from the roof and had Bi'ah with her, or was drunk and not aware of anything or sleeping, he did not acquire.

Rambam and Rosh (ibid.): If he intended to stick his Ever in the wall and it landed in his Yevamah, he did not acquire her. If he intended to stick it in an animal and it landed in his Yevamah, he acquired her, since he intended for a type of Bi'ah.

Magid Mishneh: The Rambam learns that someone drunk does not acquire from one who fell from the roof or was sleeping.

Rosh (ibid.): The Gemara said that the Mishnah cannot discuss when Nochrim forced him to have Bi'ah, for Rava taught that a man is not forced regarding Arayos, since Kishuy comes only willfully. This does not mean that Nochrim threatened to kill him if he will not have Bi'ah, and he decided to have Bi'ah b'Koshi. If so, we could give a case that is Ones, i.e. they threatened to kill him if he will not have Bi'ah b'Koshi! Rather, whatever he decides to do is called l'Da'as, even if he was mortally threatened. Rather, we must say that they stuck his Ever in the Yevamah. Because the Kishuy was l'Da'as, the Yibum is considered l'Da'as. The Gemara did not say that Nochrim forced him to have Bi'ah with an Ever Mes (without Kishuy). This implies that Ever Mes does not acquire. However, perhaps our Sugya holds that Ever Mes is exempt regarding Arayos, therefore it does not acquire. Alternatively, even according to the opinion that one is liable for Arayos with an Ever Mes, Yibum requires the kind of Bi'ah that can be Mekim Shem. "Yevamah Yavo Aleha" - whether he is Shogeg or Mezid, forced or willing, he acquires her. One who did Yibum while sleeping did not acquire.

Poskim

Shulchan Aruch (EH 166:7): If a Yavam had Bi'ah with his Yevamah he acquired her, even if he was Shogeg or l'Shem Zenus or Nochrim forced his Ever into her b'Koshi, even if he was Mezid and she was Shogeges or forced, or vice-versa, whether she was asleep or awake.

Beis Shmu'el (7): Even though it is forbidden to have relations with a woman who is sleeping, he acquires her.

Shulchan Aruch (8): This is if he intended for a Bi'ah, even for Bi'ah with an animal. If he did not intend for Bi'ah at all he did not acquire, e.g. he was sleeping or drunk and not aware of anything, or he intended to stick his Ever in the wall and it landed in his Yevamah, or if he had Kishuy for his wife and fell from the roof and had Bi'ah with his Yevamah. The same applies if Nochrim stuck his Ever Mes into her.

Beis Yosef (DH v'Da): The correct text of the Tur says 'he does not acquire when he fell from the roof or if Nochrim stuck his Ever Mes into her.

Beis Shmu'el (7): If he was half-awake, he acquires her.

Beis Shmu'el (9): Even according to the opinion that is Mechayev for Arayos with Ever Mes, Yibum requires a kind of Bi'ah that can be Mekim Shem. Even though Lo k'Darkah cannot be Mekim Shem, it is a kind of Bi'ah that (elsewhere) can be Mekim Shem.

Taz (6): Why do we discuss one who had Kishuy for his wife and fell from the roof? Since he did not intend for Bi'ah, he should not acquire even if he had Kishuy for his Yevamah! If he acquires in this case, why did the Gemara need to change the case and say that she grabbed his Ever? Rather, we are concerned only for the Biah, not for Kavanas Kishuy. We merely discuss a common case.