Modern American marriage customs and divorce are central themes in Wharton's novel. Charles Bowen, a character who acts as a social commentator, notes that it is "the custom of the country" for men to toil tirelessly to fund "his wife's extravagances" without ever discussing his work with her. This results in a lack of shared life experiences in many American marriages. The man's primary focus, the business world, remains a mystery to his wife. Meanwhile, her life revolves around a social scene of lavish display, which becomes a significant financial burden when business struggles arise. Undine expresses her perspective on American marriages early in the story by noting that her friend Mabel Lipscomb will likely soon divorce because her husband has "been a disappointment to her." Mr. Lipscomb does not fit into "the right set," and Mabel will "never get anywhere till she gets rid of him." Undine's words clearly convey her view of marriage, which remains consistent throughout the novel. To her, a husband is merely a means to an end, and if he fails to serve that purpose, a savvy woman will part ways with him.

Undine lives in a society where appearances are often mistaken for genuine identity. Her primary asset in securing a husband is her physical beauty. The novel presents a similar scenario twice, where Undine captivates the interest of two sophisticated young men—one American and the other French—hailing from respected families, solely because of her looks. These men appeal to her because they are part of the "right set" and present themselves well in social circles. She believes that marrying them will grant her access to the highest levels of society. However, the issue is that neither man has the financial means to maintain her lifestyle. Additionally, both have artistic ambitions (Marvell aspires to be a writer, while Chelles is interested in painting and music), which Undine has no appreciation for. Her perception of art is best exemplified by the society portrait artist Claud Walsingham Popple, who specializes in "idealizing flesh and realizing dress fabrics"; his portraits render his subjects uniformly elegant and "pleasing." Undine loses both husbands when they realize that the stunning beauty they admired hides a cold heart and a mind "as lacking in beauty and mystery as the prairie schoolhouse where she was educated."