A Jewish wedding is made up of two parts – kiddushin (betrothal) and nesu’im (marriage). Although it is called betrothal, kiddushin is not just a commitment to marry – it is actual marriage. For example, if the couple chooses not to complete the marriage with nesu’im, they will need a formal get (divorce). In the time of the Mishna, these two parts normally took place about a year apart, in order to give the bride and groom time to prepare for the wedding and for their marriage, but today they are done one after the other at the wedding ceremony.
One of the reasons that the tradition changed is concern over the period during which the couple is married but not yet living together.
Our Mishna presents a uniquely disturbing case that stems from this situation. Two men offer kiddushin to two women, but at the time of the nesu’im the women are switched and each ends up sleeping with the wrong spouse. In such a case, they would all need to bring a korban hatat – a sin-offering. The Mishna adds that there may be reason to obligate the parties involved in more than one korban hatat, if, for example, the men were brothers or the women were sisters.
In any case, assuming that the exchange was accidental (the Gemara rejects the possibility that the case in the Mishna could be talking about people who switched spouses on purpose, saying midei be-reshi’ei askinan!? – does the Mishna talk about evil doers!?), the proper wives would be returned to their respective husbands and obligated to wait a three month period, in order to ascertain whose children the women are pregnant with, should it turn out that one or both of them conceive. The three months – a period called havhana (literally “a period of distinction”) – are essential, both to clarify who is the true father and to determine whether the baby who is born is to be considered a mamzer – a child born from an adulterous or incestuous relationship.