Calixta and Clarisse act as foils in the story, meaning that, when contrasting them with each other, readers learn more about each of them (and about the intended takeaways of the story as a whole). In the simple act of positioning Calixta as Alcée’s lover and Clarisse as his wife, Chopin is inherently encouraging readers to view them in juxtaposition.
The major difference between the women is that Calixta feels rejuvenated by sex with Alcée while Clarisse feels rejuvenated by not having sex with him. This comes across in the final section of the story (the first part in which Clarisse directly appears):
[T]he first free breath since her marriage seemed to restore the pleasant liberty of her maiden days. Devoted as she was to her husband, their intimate conjugal life was something which she was more than willing to forego for a while.