Sabina feels as though Franz has destroyed their privacy. She has no interest in being viewed publicly as Marie-Claude's rival.
Now that her life is public, she has to start cultivating a public image, which means living in lies.
But she assures Franz that she is not angry. That night, she turns off the light before getting into bed with him. This should have tipped him off that something was wrong, but he doesn't notice (because he makes love with his eyes closed anyway).
Sabina couldn't stand to look at his closed eyes any longer. When he closed his eyes, he became just a body without a soul, and this disgusted her.
Sabina knows that Franz is the best, kindest man she's ever had. But she wants to defile his kindness.
That night they make love and she feels a longing for betrayal, for freedom.
The narrator points out that both are "drunk on betrayal" – Sabina for betraying Franz and Franz for betraying his wife (3.8.14).