Yevamot 54A

ישן לא קנה ביבמתו. אלא בנתקע ⁠— והא אמר רבה: נפל מן הגג ונתקע ⁠— חייב בארבעה דברים, וביבמתו לא קנה.

a sleeping man has not acquired his yevama, as he did not intend to perform the act of sex at all? Rather, the Mishnah was referring to one who was inserted into his yevama by accident. But didn’t Rabba say: One who fell from a roof and was inserted into a woman due to the force of his fall is liable to pay 4 of the 5 types of indemnity that must be paid by one who damaged another: Injury, pain, medical costs, and loss of livelihood. However, he is not liable to pay for the shame he caused her, as he did not intend to perform the act, and if she is his yevama, he has not acquired her in this manner.

אלא כגון שנתכוון לאשתו, ותקפתו יבמתו, ובא עליה. שניהם אנוסים דבי רבי חייא היכי דמי? כגון שנתכוון לאשתו, ותקפוהו גוים ודבקום זה בזה, ובא עליה.

Rather, it is a case where he intended to have sex with his wife and became erect, and his yevama forcefully grabbed hold of him and he had sex with her. The Talmud further asks: If so, what are the circumstances of the case when both of them were coerced that was mentioned by the school of R' Ḥiyya? The Talmud answers: It is a case where he intended to have sex with his wife, and non-Jews grabbed hold of him and pressed him and his yevama against each other, and he thereby had sex with her.

מנא הני מילי? דתנו רבנן: ״יבמה יבא עליה״, מצוה. דבר אחר: ״יבמה יבא עליה״⁠ — בין בשוגג בין במזיד, בין באונס בין ברצון.

§ The Talmud inquires as to the source of these halakhot: From where are these matters derived? As A baraita states with regard to the verse “Her brother-in-law will have sex with her” (Deuteronomy 25:5), that this indicates that the act of sex in this circumstance is a mitzva, i.e., it is preferable to the alternative, which is ḥalitza. Alternatively, the verse “Her brother-in-law will have sex with her,” indicates that it does not matter how he had sex with her, whether unwittingly or intentionally, whether due to coercion or willingly.