As we have seen, the commandment of yibum allows the wife of a man who dies without offspring to be married by one of his surviving brothers, negating the Biblical prohibition forbidding a man from marrying his brother’s wife. What is the status of their relationship, once yibum has been performed? Are they still seen as connected because of the original marriage to the deceased brother, or are they simply a married couple?
Our Gemara quotes Rabbi Yosei bar Hanina, who understands the passage in Sefer Devarim (25:5) to mean that once the yavam takes her as his wife, all of the rules and regulations of normal marriage apply to them. Thus, should the couple choose to get divorced, rather than performing the halitza ceremony, the husband will divorce his wife with a geṭ, a standard divorce document. Furthermore, should they choose to do so, they can remarry after the divorce, without any concern that the original prohibition of marrying one’s sister-in-law still exists.
Tosafot in Yevamot ask why the Gemara only searches for a source that will allow the yevam and yevama to remarry after a divorce, without expressing any concern about their ongoing sexual relationship while they are married. They suggest that it is only the very first act of sexual relations that is the fulfillment of the mitzva of yibum; from then on their relationship is one of choice. Should the Sages not search for a Biblical passage that clearly permits the marriage to continue after the basic mitzva is fulfilled?
The answer is an issue of common sense. There is no need to find a pasuk that clearly permits this, since it would be illogical for the Torah to command someone to engage in sexual relations once and then file for divorce. Another approach is suggested by the Netziv in his Meromei Sadeh, where he argues that whenever we find that performing a mitzva pushes aside a prohibition, as long as the person remains involved in that mitzva he can add to it, even beyond the basic obligation.