]I guess she hoped that this doctrine might appeal to his sanctimonious manner, and therefore save herself the grief of dancing with him!

This is exactly what I thought. She immediately perceived that he intended to go and made her comment to him so as to, in the best case scenario, maybe induce him to change his mind, and if that were not possible, at least she could amuse herself by hearing him scramble to defend his decision. Of course, as she notes, it backfired upon her in a terrible way!