The Mishna that opens Massekhet Kiddushin (2a) discusses not only a normal situation of marriage, but also the case of yibum. On a biblical level, yibum can only be done with bi’ah between the widow and her brother-in-law.
The passage in the Torah that is the source for the mitzva of yibum is found in Sefer (25:5), which describes how, in the event that a man dies with no children, his widow should not marry an outsider, but rather yevama yavo aleha, u-lekahah lo le-isha, ve-yibmah – the surviving brother should come upon her, and take her as a wife, fulfilling the mitzva of yibum with her.
Examples of laws that are derived from this passage include:
- that the performance of yibum is the fulfillment of a mitzva;
- that the sexual act will accomplish yibum, whether or not it was done with intent;
- that any sexual act, whether “natural” or “unnatural,” will complete the yibum.
Based on the first principle mentioned above, several poskim rule that the yavam (the surviving brother) should make a blessing before performing yibum, since the performance of every mitzva requires a berakha beforehand (see Shulḥan Aruk, Even ha-Ezer 166).
In answer to the question of why we need a specific teaching for the idea that any sexual act whether “natural” or “unnatural” will complete the yibum, given the general principle that halakha always treats any act of sexual intercourse as having halakhic significance, the Ramban suggests that we may have thought that yibum should be an exception to that rule. Given that yibum focuses on continuing the name of the brother who had passed away, we may have thought that only a sexual act that potentially could have led to pregnancy would have been significant. Thus we need to be taught that even in the case of yibum any sexual act will suffice to fulfill the mitzva.