The Storm Steaminess Rating
The language Chopin uses may be a little coy and full of figurative language: "an immaculate dove" (2.18), "firm, elastic flesh...like a creamy lily" (2.19), and so forth. It isn't crude or vulgar; the most explicit language is used in reference to Calixta's breasts. The author restricts her description of the bodies to the waist up, using vaguer language to describe sexual actions and pleasure. In other words, for 21st century readers this story falls squarely in R-rated territory.
By 1890s standards, though, it probably was equivalent to whatever they thought of as NC-17 back then: forbidden, naughty, and full of dangerous desire. Not only does the story advocate the enjoyment of sex, but that sex is outside of marriage. And not only that, the author seems to condone the adultery. The characters aren't punished, and in the end "every one was happy." These aren't the typical morals of 19th century writing.