Tomas wakes Tereza up in the middle of the night because she's moaning in her sleep.
She tells him about her dream; she was with him and Sabina in a room with a large bed in the middle, like a platform in a theatre. Tomas made her stand in the corner while he made love to Sabina. Tereza, to distract herself, shoved needles under her fingernails.
The next day, Tomas realizes that Sabina wrote about a similar fantasy (making love to him on a bed on a platform like in a theater, while others watched) in a letter. He realizes that Tereza went through his stuff and confronts her about it.
She admits it, but is still angry at him for cheating.
Tomas's incessant womanizing becomes a major issue. He insists that having sex with other women in no way compromises his love for Tereza.
Tereza is a nervous wreck; the infidelities are taking their toll on her.
Tomas also recognizes that their relationship is totally unequal; he would never let her sleep with another man. He even gets jealous when she dances with another man while they're out one night, as he realizes that she might just as easily have given herself to another man instead of to him (Tomas).