Gemara

53b - Question: What is the case when he was forced (to do Yibum)?

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, 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 in this case 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: How can we find a case in which they are both forced?

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

Sanhedrin 74a (Beraisa - Rebbi) Question: "Ki Ka'asher...u'Retzacho Nefesh" connotes that we learn the law of Na'arah Me'orasah from that of a murderer. But it says explicitly that we do not kill her because she was forced!

Answer: Just like one must forfeit his life and not murder, one must forfeit his life and not transgress Arayos.

Question: Esther was married to Achashverosh in public (i.e. many Yisraelim knew about it. She should have refused and forfeited her life!)

Answer #1 (Abaye): She was Karka Olam (passive, until she went to the king on Mordechai's command. Therefore, Mesiras Nefesh was not required.)

Answer #2 (Rava): If a Nochri wants a Yisrael to transgress for the Nochri's benefit, even in public he need not forfeit his life.

Rishonim

The Rif and Rosh (Sanhedrin 17a and 8:3) bring the ruling of R. Yochanan.

Rambam (Hilchos Isurei Bi'ah1:9): Ones is exempt from lashes and Korban and we need not say from death - "Vela'Na'arah Lo Sa'aseh Davar". This is when the Niv'al (the one in whom the Ever was inserted) was forced.

Source (Magid Mishneh): The Sifri exempts from death, Korban and lashes from "Vela'Na'arah Lo Sa'aseh Davar", "Chet" and "Maves".

Rambam (ibid.): Ones does not apply to the Bo'el, for Kishuy comes only willingly. If a woman was initially forced and willing at the end, she is totally exempt. Once a man started to have Bi'ah with her, she cannot avoid desiring it. A person's Yetzer and nature force him to want.

Ra'avad: There is a case of Bo'el b'Ones, when he intended for his wife (and had Kishuy) and one of the Arayos put his Ever to her Ervah, or Nochrim stuck it there.

Migdal Oz: The Rambam had the text of the Rif in Sanhedrin (74a), 'just like one must forfeit his life and not murder, one must forfeit her life and not transgress Arayos.' It is feminine because there is no case of Ones regarding a man. The Ra'avad's text reads 'one must forfeit his life', i.e. and not do an act. A woman should not be Moser Nefesh, for she is passive, like the Gemara says about Esther. Even if the text says 'her life', this refers to when she is commanded to do an act

Magid Mishneh: Some, including the Rambam, hold that if one was Bo'el an Ervah due to a mortal threat, he is killed, because Kishuy comes only willingly. Others say that since the Kishuy was due to the threat, he is exempt b'Yedei Adam. Rava discusses when Nochrim stuck his Ever to the Yevamah without threatening him. Why didn't we establish the Mishnah in such a case? Because the Bi'ah itself was not b'Ones, it is not appropriate to call this 'Ba b'Ones'. The Rambam agrees that the Ra'avad's case is Ones.

Sefer ha'Chinuch (Mitzvah 556): If one transgressed one of the three Aveiros due to Ones, even though he was obligated to be Moser Nefesh and not transgress, Beis Din does not kill him. The exception is if a man was Bo'el Ervah, for this is not Ones, since Kishuy comes only willingly.

Rebuttal (Tosfos 54a DH Ein): Even though a woman is liable when she is Niv'al willingly, "Vela'Na'arah Lo Sa'aseh Davar" exempts Ones, even if she did an act. The same applies to a man, even though he should be Moser Nefesh.

Rebuttal (Yam Shel Shlomo, ibid.): Since a woman is passive in Bi'ah itself, she is exempt even if she does an act due to Ones. This does not apply to men.

Rambam (Hilchos Sanhedrin 20:2): Anyone who transgresses Chayavei Misas Beis Din b'Ones is not killed. Even if he was commanded to submit to be killed, even though he was Mechalel Hash-m, he is not killed. "Vela'Na'arah Lo Sa'aseh Davar" warns Beis Din not to punish one who was b'Ones.

Rambam (3): If a man was forced to have Bi'ah with Ervah he is Chayav Misas Beis Din, because Kishuy comes only willingly.

Rebuttal (ha'Ramach): Why is this worse than serving idolatry from love or fear, for which one is exempt? Also, one is not killed unless he was warned and said that he consents to be transgress and be killed!

Answer #1 (Kesef Mishneh): Why is this worse than the previous Halachah, in which the Rambam exempts even one who did an act? Perhaps the act was done unwillingly, but Kishuy comes only if he desires. If he had Kishuy for his wife and was forced to have Bi'ah, he is exempt. If they forced him to have Kishuy and stuck him on Ervah he is liable. This requires investigation. Perhaps the Rambam means that he is like one who transgressed willingly. He does not say that Beis Din kills him, just that he is Chayav Misas Beis Din.

Rebuttal (Yam Shel Shlomo 6:2): The Rosh (in his Perush on SMaG Lav 201) says that if a man transgressed Arayos b'Ones he is worthy to be killed, but Beis Din does not kill him. This is difficult. If so, the Rambam should have said that he is Chayav Misah b'Yedei Shomayim! Also, if the Ones helps to exempt from Misah, how did we prove from Rava that Ein Ones b'Arayos?!

Answer #2 (Chelkas Mechokek EH 20:3): The Rambam discusses when he was forced to have Bi'ah, but there was no coercion to have Kishuy.

Question: Rava implies that if Kishuy came unwillingly, one would be Ones and not need to be Moser Nefesh. Mesiras Nefesh is required for Arayos!

Answer (Rosh Yevamos 6:1): Mesiras Nefesh is not required when one is Karka Olam, e.g. if one will be thrown on a baby to kill the baby. Also regarding Arayos Karka Olam exempts, e.g. a woman, or a man already in Kishuy (if he does not move). However, a man without Kishuy will surely come to Kishuy if put on Ervah. This is a willful action. Even if the Halachah is Mechayev for Arayos without Kishuy, causing Kishuy is an act to increase his pleasure, so he is not considered Ones.

Rebuttal (Yam Shel Shlomo 6:2): If so, why does the Gemara answer that his Yevamah grabbed his Ever? We could say that Nochri forced him (after he had Kishuy for his wife), like we originally thought! Also, how can we ask against Rava from Yibum? Perhaps there is no Ones only regarding Arayos, for which one must be Moser Nefesh, but not regarding Yibum! Rather, whenever another party physically forces, both of them are Anusim. To explain when he is b'Ones but she is not, we said that Nochrim forced him, i.e. by threatening to kill him. We rejected this because Rava taught that Kishuy is always intentional. He is coerced to do a Mitzvah, and he intends to do it properly, so we cannot call this Ones. The Mishnah would not need to teach that he acquires! Therefore, we answer that she grabbed his Ever.

Rambam (Hilchos Yesodei ha'Torah 5:1,2): All of Yisrael are commanded about Kidush Hash-m. If one is told to transgress Arayos or be killed, he must submit to be killed.

Question: Esther was "l'Vas" (a wife) to Mordechai. Why did the Gemara ask only why she was not Moser Nefesh for Kidush Hash-m? Also, she should have been Moser Nefesh for Arayos!

Answer #1 (Rosh, ibid.): Esther did not do an action. The Gemara knew that passivity exempts from Mesiras Nefesh for Arayos. It thought that this does not exempt from Mesiras Nefesh to avoid Chilul Hash-m.

Answer #2 (Ba'al ha'Ma'or Sanhedrin 17b): Mesiras Nefesh is never required when the Nochri intends for his own benefit. We learn from Esther. This was in public and it was Arayos. The Gemara preferred to ask why Esther was not Moser Nefesh for Kidush Hash-m, as opposed to for Arayos, for it is not clear that she was married to Mordechai. One must be Moser Nefesh for Na'arah Me'orasah only when others force him to transgress, but not if she herself forces him for her own desire. The Yerushalmi and the She'altos support this.

Milchamos Hash-m: The opinion that Esther was married to Mordechai must hold like Abaye, who permits because of Karka Olam. Rava must hold that she was not married to Mordechai. Alternatively, even though she was married, Bi'ah with a Nochri is not considered Arayos.

Poskim

Shulchan Aruch (YD 157:1): If a Yisrael is told to transgress Arayos (or idolatry or murder) or be killed, he must be Moser Nefesh. This is even in private when there is no decree, even if the Nochri intends for his own benefit, one must be Moser Nefesh.

Chelkas Mechokek (20:3): According to the Mordechai and Tosfos, when a man was coerced into adultery, even though he should have been Moser Nefesh, he is not killed. The Bach says that therefore, she is not forbidden to her husband. This is difficult. If she was forced, surely she is permitted if her husband is a Yisrael! Perhaps he means that if she was threatened to do an act, according to those who say that she is not killed she is not forbidden.

Note: Perhaps the Bach holds that the Bo'el is not forbidden if he was b'Ones, and she is forbidden to her husband only when she is forbidden to the Bo'el!

Rema: This is only if they want the Yisrael to do an act, e.g. to have relations with a woman. But if they forcibly have relations with a Yisraelis, or if a man was in Kishuy they want to force his Ever onto an Ervah, Nisuch ha'Mayim is not required. It is required for any transgression pertaining to Arayos, even a Lav (without Misah), but not for "V'Lifnei Iver Lo Siten Michshol". Bi'as Nochri with a Bas Yisrael is not considered Arayos.