After Mordechai found out about Haman's decree, he sent a copy of it to Esther, along with instructions that she beseech Achashveirosh on behalf of her nation. She initially demurred, explaining that it is forbidden to appear before the king without being summoned. Mordechai warned her against remaining silent, and told her that perhaps it was for this reason that Hashem placed her in this position. Esther relented and told Mordechai to summon the Jewish people to fast on her behalf, and after three days, she would go in unlawfully to Achashveirosh, adding, “And if I perish, I perish.”
On a literal level, Esther was referring to the possibility that Achashveirosh may have her put to death for entering his inner court without being called, but the Gemara (Megillah 15a) interprets her words as lamenting the fact that until this point, each time that she had relations with Achashveirosh it was against her will. Now that she was going in to him voluntarily, it would be considered as if she was a willing participant. The halachah is that a married woman who voluntarily has extramarital relations becomes forbidden to remain married to her husband (Yevamos 56b). Even after Esther married Achashveirosh, she remained married to Mordechai and continued secretly having relations with him (Megillah 13b). However, this was only permitted as long as her interactions with Achashveirosh were against her will. Now that she was willingly going in to have relations with him, she would be forbidden to Mordechai for the rest of her life.
Rav Dovid Feinstein points out that this is yet another example of hester panim [Hashem's seemingly concealed face] in the Megillah. Esther did exactly what Mordechai commanded her and told her was necessary to save the entire Jewish nation, and yet the very same action which was considered one of the greatest mitzvos in Jewish history caused her to become permanently forbidden to her husband.
However, Tosfos questions why Mordechai didn’t divorce Esther at this time, as any relations that she would have while single - even voluntarily - would not prevent her from subsequently remarrying him. Tosfos answers that giving one’s wife a get [divorce document] must be done in the presence of two witnesses. Mordechai feared that the witnesses might absentmindedly talk about the event, which would eventually make its way to the ears of Achashveirosh, thereby endangering the lives of himself, Esther, and the entire nation. However, the Rashba questions why Mordechai didn't give Esther a get written in his own handwriting, which according to many opinions effects a divorce even in the absence of witnesses. The Rashba cryptically answers: אין משיבין על דברי אגדה – “It is not always possible to ask or answer questions regarding homiletic passages”.
The Noda B’Yehudah (Yoreh Deah 2:161) was once asked to resolve a difficulty with a different homiletic passage. He begins his reply by describing how pressed he is for time to answer even the most pressing and practical questions which come before him and apologizes that he is unable to delve into the esoteric subject at that time. He further justifies his actions by quoting the aforementioned Rashba, who writes that such questions can’t always be asked and may not have readily apparent resolutions. The overworked Noda B’Yehuda then continues, “But now that I've mentioned the Rashba’s comment, let me tell you the answer to his question!” He suggests that as a historical fact, Mordechai did indeed divorce Esther without witnesses by using a get that he personally wrote, exactly as suggested by the Rashba.
If so, why does the Gemara understand Esther as lamenting the fact that she would be eternally forbidden to Mordechai, which according to the Noda B’Yehudah should not be the case? He innovatively explains that while it was possible for Mordechai to divorce Esther without attracting attention, thereby preventing her from becoming forbidden to him, there was no parallel option to subsequently remarry her. A wedding must be conducted in the presence of witnesses to take effect, leaving Esther technically permitted to Mordechai, but forbidden for all practical purposes. (R’ Ozer Alport)
