As we have learned, according to the Torah, the basic requirement to fulfill the mitzva of yibum is for the surviving brother (the yavam) and the widow (the yevama) to engage in bi’ah – an act of sexual relations – in order to signify the “continuation” of the original marriage. Given the central role played by bi’ah in the fulfillment of this mitzva, it become essential that we define our terms, both as far as the act itself is concerned, but also with regard to the intention of the participants in this act. Must they intend to fulfill a mitzva with this bi’ah, or is the sexual act itself enough?
The first Mishna in the perek opens with the simple assertion that a sexual act between a yavam and a yevama will be effective, even if either one – or both – participated accidentally, or even if they were forced to do so. This is true whether or not there is full penetration, with no distinction between one act of bi’ah and another. The Mishna concludes that these rules are true not only in the case of yibum, but also in other laws that involve issues of sex, including cases of erva (adulterous or incestuous relationships) and those involving less severe forbidden sexual relationships (e.g. a kohen marrying a divorcee).
Rabbi Yosef Rapp, in his Yosef Lekah, asks why these rules are examined in Massekhet Yevamot. He argues that it would have been more logical to have listed and examined them in Massekhet Makkot, where the laws of forbidden sexual relationships are discussed, and then applied those rulings to the laws of yibum, as well. He answers that yibum has a number of unique issues that could not be understood from a comparison to forbidden sexual acts. For example, 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.