We know it's a cliché, but in this story actions speak louder than words. Calixta cheats on her husband. Alcée cheats on his wife. Both experience a pleasure unlike anything they've ever known before. Both seem to find something essential in their affair that's missing in their marriages. Both commit – without saying anything to each other, it seems – to keeping their affair a secret.
Afterward, Alcée and Calixta both seem to try to overcompensate within their own marriages. Calixta gathers her husband and son up and in, gushing over them and trying to show how happy they all are. In contrast, Alcée writes to his wife from a distance, formally expressing his love for her and giving her permission to stay away longer. Calixta draws her family in, Alcée pushes his away. But they're both reacting to the same thing: the greatest sex of their lives.
Sex and Love
Sex is a defining characteristic for many of the folks in this story. By having great sex with each other, Calixta and Alcée also reveal key elements about their sexual relationships with their spouses. The fact that we don't know much about Bobinôt's approach to sexuality is significant: the most we learn about him in bed is that he doesn't quite do it for Calixta. Calixta has the best sex of her life during the illicit affair with Alcée and feels "for the first time" (2.19) what her body is capable of.
Similarly, Alcée seems to achieve levels of sensuality he'd never imagined, giving a "response in depths of his own sensuous nature that had never yet been reached" (2.20). Like Calixta, Alcée doesn't seem to feel the same way with his spouse. Clarisse is relieved to have some time apart from him, specifically to spend some time outside their marriage bed.
While Calixta and Alcée might well love their spouses more than they love each other, they definitely have far better sex outside of marriage. The story doesn't clearly explain how connected love is to sex for either of these couples. It's up to the reader to determine whether Calixta and Alcée can have such great sex because they still love each other or in spite of the fact that they love other people (or both).