She not only saw them leave, she watched him in action naked from the open window over his bed. He left it unlatched for her. Remember their very first scene? He wanted her to watch him, because he wanted her to see him as a man and desire him like he desired her. He was all over her right after, smelling of sex, and she didn't mind. They were looking at each other's lips, wanting to kiss.
Then later in church, she teased him saying "surely marriage is good" when he told her she'd never marry if he could help it. They both were playing the want & wanting game
