Francis wants to get the truth out of Micheal so that she can confirm what she seems to have already know: that Micheal lusts after other women. This demand for the truth does not come until the end of the story when Francis has become sick of Micheal's equivocating. This can be seen in the following exchange:
"You'd like to be free to ..." Frances said.
"Tell the truth." She took her hand away from under his.
Michael flicked the edge of his glass with his finger. "Okay," he said gently. "Sometimes I feel I would like to be free."
Here, Frances forces Michael to speak the truth that both of them already know and to put an end to their passive-aggressive forms of argumentation.
Earlier in the story, Frances addresses her husband's behavior by making a simple joke; that is, whenever she sees Michael looking at another woman, she humorously states: "You'll break your neck." Though they both laugh at the joke, it quickly becomes clear that there is real pain underneath Frances' jest and that Michael's behavior is causing her emotional turmoil.
The breaking point comes when Frances catches Michael looking at another woman as she is trying to plan a romantic day trip for the two of them. Finally, Frances decides to be direct: "'You always look at other women,' Frances said. 'At every damn woman in the city of New York.'" By directly confronting Michael's persistently wandering gaze, Frances eventually gets Michael to speak out loud about his desires, though the end of the story is ambiguous about what will become of their relationship.