Rabbenu Tam and the Case of Gentile Adultery

 https://www.etzion.org.il/en/rabbenu-tam-and-case-gentile-adultery

 The gemara in Sanhedrin (74b) questions the appropriateness of Esther’s behavior, and particularly her willingness to marry Achashverosh even though she was married to Mordechai (according to Chazal).  In questioning her behavior, the gemara claims that her behavior was unacceptable because the public was aware of her conduct.  Even minor mitzvot must be kept under pain of death if their commission is public knowledge.  Although the gemara’s question is relevant, Esther’s conduct could have been questioned on more basic grounds - she was committing giluy arayot!! This conduct must be avoided EVEN IN PRIVATE.  Why was the gemara only concerned with the public nature of her conduct and not with her decision to violate giluy arayot?