The early sages ruled that a sexual act as the way to acquire a woman is legal and valid even if one of the sides is forced. For example, if the levir (brother-in-law) raped the widow of his brother, the levirate marriage is valid. The scholars asked: How can the brother-in-law be forced to perform sexually? Answer: For example, gentiles came and forced him to have sexual relations with his brother’s widow. The scholars rejected this example because one of the sages, Rava, ruled that a man cannot achieve an erection if he is forced to, for his sexual organ will only become erect by his own will. Therefore, even if gentiles forced him to have sexual relations with his brother’s widow and he did achieve an erection, it is considered as though he desired this and not that he was forced. The scholars argued that the brother-in-law is considered forced when he has sexual relations with his sister-in-law as he sleeps. (A sleeping man often has erections and he would not know that he had had sexual relations, so he could be considered forced.) The scholars asked: Did not another sage, Rav Judah, state that acquiring through sexual relations as a man sleeps is not a valid or legal levirate marriage, for the brother-in-law must be aware of the sexual contact and know that he is making such contact? The scholars argued that the brother-in-law is considered forced when his sexual organ penetrates her without his intention. The scholars asked: Did not another sage, Rava, rule that a levir who falls from a roof and penetrates the sexual organ of his brother’s widow, who had been lying on the ground with spread legs, does not legally acquire her? The reason is that he must intend the act and know that he will penetrate the sexual organ of the woman he is supposed to take in a levirate marriage. The scholars argued that the brother-in-law is considered forced when he has sexual relations with his sister-in-law in the following way: he meant to have sexual relations with his own wife and suddenly his sister-in-law attacks him and forces him to have sex with her. A different sage, R’ Hiyya, argued that even if both sides (the levir and the sister-in-law) are forced, the act of sexual relations is a legal and valid act of acquirement. How could it be that both are forced? The levir intended to have sexual relations with his own wife and gentiles came and made his sexual organ penetrate his sister-in-law’s by force. The scholars then tried to clarify the rule for a levir who has sexual relations with his brother’s widow while he is asleep, for above it was claimed in the name of Rav Judah that such an acquisition is not valid. The early sages, though, said that one who has sexual relations with his brother’s widow while asleep does acquire her. Answer: The early sages permitted this acquisition if he has sexual relations with his brother’s widow while half asleep, but not while fully asleep. What does “half asleep” mean? If one calls him he answers, but when he is asked a question he does not remember it unless prompted. The sage Rava added that if the levir intended to masturbate into a hole in the wall but ended up unknowingly penetrating his sister-in-law’s sexual organ, the sexual act is not a valid acquisition. But if the levir intended to have sexual relations with an animal but in the end has sexual relations with his brother’s widow, the act of acquisition is legal and valid. What is the difference between the two acts? Answer: One who masturbates into a hole in the wall did not intend sexual relations with a partner, while one who intended sexual relations with an animal did.
(Babylonian Talmud, Tractate Yevamot 53b-54a)