The Gemara answers: That stands to reason, since betrothal takes effect with those women who are forbidden and with whom he would be liable for the violation of a prohibition. That is, if a man betroths a woman who is forbidden to him and with whom he would be liable for the violation of a prohibition, then although he violates a prohibition in doing so, the betrothal is valid and cannot be ended without a bill of divorce. Therefore, such a woman also requires ḥalitza. In contrast, betrothal does not take effect at all with those who are forbidden and would be liable to receive the punishment of karet, and therefore in these cases the laws of levirate marriage and ḥalitza do not apply at all.

Rava raised an objection to the explanation of Rav: It is taught in a baraita with regard to a prohibition resulting from a mitzva and a prohibition stemming from sanctity that if he engages in intercourse with such a woman or performs ḥalitza with her, her rival wife is exempt, even though it was prohibited for him to have engaged in intercourse with her in the first place. If it enters your mind that women who are forbidden, as he would be liable for the violation of a prohibition, require ḥalitza by Torah law but do not require levirate marriage, then when he engages in intercourse with his yevama why is her rival wife exempt? If there is no biblical mitzva to engage in intercourse with her, his action would carry no halakhic validity and the rival wife should not be exempt.

Rava raised the objection and he resolved it: The baraita teaches it disjunctively; it did not all deal with the same case. When the baraita says: Engages in intercourse with her, it is referring to a prohibition resulting from a mitzva. If one engages in intercourse with a yevama prohibited to him by rabbinic law, since by Torah law levirate marriage with her is valid, then although his act involved the transgression of a rabbinic decree, he nevertheless fulfilled the Torah mitzva and the rival wife is thereby exempt. When the baraita says: Performs ḥalitza with her, it is referring to a prohibition stemming from sanctity, and by Torah law there is no option of levirate marriage because of the prohibited relation; therefore, only ḥalitza exempts her rival wife.

Rava raised an objection from that which was taught in the Tosefta (Yevamot 11:3): A man with crushed testicles or with other wounds to his genitals or one whose penis has been severed, one who is a eunuch caused by man and not from birth or by disease, or an elderly man, all of whom are incapable of fathering children, one either performs ḥalitza or levirate marriage. How so? If any of these infertile men died, and they had brothers and they also had wives, and they then died childless, and the brothers proceeded to perform levirate betrothal with their wives, or gave them a bill of divorce, or performed ḥalitza, whatever they did is done; i.e., their act was effective. And if any one of the brothers engaged in intercourse with the widow of one of those infertile men, he thereby acquired the woman as a wife according to the laws of levirate marriage.

The inverse is also true: If the brothers died childless, and the infertile men proceeded to perform levirate betrothal with their wives, or gave a bill of divorce, or performed ḥalitza, whatever they did is done and took effect. And if they engaged in intercourse with their yevama, they thereby acquired the yevama as their wife. However, it is forbidden to maintain them, i.e., allow them to continue to live as husband and wife, because it is stated: “One with crushed testicles or whose penis has been severed shall not enter into the assembly of the Lord” (Deuteronomy 23:2); they are prohibited from entering the congregation, i.e., marrying a Jew. And if it enters your mind that women who are forbidden, as he would be liable for the violation of a prohibition, require ḥalitza by Torah law but do not require levirate marriage, then one could ask: if they engaged in intercourse why are they acquired as wives even though there would be no mitzva of levirate marriage because the men are prohibited from marrying them?

Rather, Rav’s opinion is rejected, and Rava said an alternative explanation: The reason why a High Priest does not take a widow from betrothal in levirate marriage is because that relationship is also a violation of both a positive mitzva and a prohibition and therefore a different positive mitzva does not override it. How so? As it is written: “They shall be sacred to their God” (Leviticus 21:6), which teaches that there is a positive mitzva of sanctity associated with all prohibitions applying to priests. Therefore, any such prohibition contains both a positive and a negative mitzva.

The Gemara asks: This resolves the issue of priestly prohibitions, but what is there to say about a daughter born from an incestuous or adulterous relationship [mamzeret] or a Gibeonite woman, who are prohibited from entering the congregation due to considerations of sanctity? They too may not enter into levirate marriage despite the positive mitzva, which would ordinarily override a prohibition. The Gemara answers: It is written with regard to all of the mitzvot: “Sanctify yourselves, therefore, and be sacred” (Leviticus 11:44). This teaches that in addition to the prohibition, there is the positive mitzva of sanctity.

The Gemara raises an objection: If so, then every single prohibition in the entire Torah contains both a positive mitzva and a prohibition, as it is written: “Sanctify yourselves” (Leviticus 11:44). Rather, this reasoning must be rejected, and Rava stated a different reason: While in essence the mitzva of levirate marriage does apply here, nevertheless, a widow from betrothal is prohibited from entering into levirate marriage with the High Priest by rabbinic decree, due to the case of a widow from marriage.

The Gemara asks: What is there to say about the case of a mamzeret or a Gibeonite woman? There appears to be no reason for a rabbinic decree in such cases. The Gemara answers: There, one must say that intercourse with a mamzeret even when the mitzva of levirate marriage applies was prohibited by rabbinic decree due to cases when the mitzva of levirate marriage does not apply. The decree was issued lest one come to think that since in the case of levirate marriage a mamzeret is permitted, even in cases when there is no levirate marriage a mamzeret is similarly permitted.

The Gemara asks: However, if that is so, and the levirate marriage is prohibited lest it become confused with another case, then the wife of a paternal brother should not enter into levirate marriage; i.e., by the same logic, although the Torah allowed it, the Sages should have established a rabbinic decree requiring that she perform ḥalitza due to the case of the wife of a maternal brother, who always remains prohibited as a brother’s wife. The Gemara answers: The Merciful One made levirate marriage dependent upon inheritance, and it is well known by everyone that only patrilineal relatives inherit, so there is no likelihood of confusion.

The Gemara objects further: Then a childless woman should not enter into levirate marriage even though the mitzva applies to her; there should be a rabbinic decree due to the case of a woman who has children. The Gemara answers: The Merciful One made levirate marriage dependent upon children; it is well known by everyone that the entire purpose of levirate marriage is to establish one’s brother’s name and that levirate marriage applies only when there are no children. Here, too, there is no likelihood of error.

The Gemara challenges further: The wife of a brother with whom one did coexist should not enter into levirate marriage; there should be a rabbinic decree due to the case of the wife of a brother with whom he did not coexist. The Gemara responds: The Merciful One made levirate marriage dependent upon a common dwelling together and coexistence of brothers, and this is well known by everyone since the matter is explicit in the Torah.

The Gemara continues to object: No woman should enter into levirate marriage; there should be a rabbinic decree due to the case of an aylonit. Since an aylonit may not enter into levirate marriage, all other women should be prohibited by rabbinic decree from doing so to avoid confusion. The Gemara answers: The case of an aylonit is not commonplace, and the Sages did not institute rabbinic decrees on matters that are not common. The Gemara asks: If so, neither a mamzeret nor a Gibeonite woman is commonplace either. Therefore, since the likelihood of taking a mamzeret in levirate marriage is so small, there is no danger that one might think it is permitted to marry a mamzeret even where the mitzva does not apply.

Rather, Rava said that it is necessary to reject the previous suggestion and to offer a different reason: The first act of intercourse is prohibited by rabbinic decree due to the likelihood of a second act of intercourse. Although intercourse the first time with the yevama is the fulfillment of a positive mitzva, which does override the prohibition, once the mitzva is fulfilled with that act there is no longer any positive mitzva involved. Afterward, this yevama becomes prohibited because there is no longer a positive mitzva to override the prohibition. Therefore, due to the possibility that one might engage in intercourse a second time with this woman, the Sages decreed that even the first act is prohibited.

The Gemara comments: This is also taught in a baraita: If one of those yevamin who may not marry their yevama due to a prohibition engaged in intercourse with her, he acquired her with the first act of intercourse; however, it is prohibited to retain her for a second act of intercourse.

The Gemara continues with a retraction from Rava: Rava then said, and some say it was actually Rav Ashi who said: That which I said, that the reason for the rabbinic decree was to prevent a second act of intercourse, is not correct, as there is a simpler explanation. As Reish Lakish said about the same matter: In every place that you find a positive mitzva and a prohibition applying to the same matter, if you can fulfill both of them together, this is best, and the positive mitzva does not override the prohibition. And if there is not any possibility of fulfilling both, then let the positive mitzva come and override the prohibition. Here, too, in the case of levirate marriage, it is possible, by way of ḥalitza, to fulfill the positive mitzva and not to transgress the prohibition prohibiting marriage to these women.

The Gemara raises an objection to this last statement by Rava from that which is taught in a baraita: And if one of those yevamin engaged in intercourse, he acquired her as a wife. This shows that although it is possible to perform the mitzva by way of ḥalitza, if he nevertheless performs levirate marriage then the positive mitzva overrides the prohibition and the yevama is thereby acquired as his wife. The Gemara concludes: This is a conclusive refutation, and Rava’s last explanation is rejected. The previous explanation is the correct one: The prohibition is due to rabbinic decree.

§ On the same subject, it was stated with regard to the case of a High Priest who engaged in intercourse with a widow who was his yevama requiring levirate marriage that the amora’im Rabbi Yoḥanan and Rabbi Elazar disputed the matter. One said that intercourse does not exempt her rival wife who had also been married to the High Priest’s brother, since the act was prohibited, and one said that it does exempt her rival wife, because although intercourse was forbidden, it is nevertheless a valid enactment of levirate marriage, and so her rival wife is thereby exempt.