Most of Wharton's work is to point out the ridiculous lifestyle of the rich and famous in her era. They do things without knowing the reasons...only because they have always been done that way. Silly, really. Wharton was a rebel in her own right, writing books for one (considered a lowly occupation for anyone of her social status, but especially for a woman). Her books include decorating advice books to social criticisms.
In The Custom of the Country, Wharton produces Undine Spragg--a beautiful, shallow, spoiled, greedy, and ambitious young woman who does not automatically bow down to the traditional "rules" of behavior for young women. She is Wharton's "new woman" character who was popping up in many of the contemporary literature examples of the time. Undine reacts negatively to becoming pregnant, neglects her child when it is born, and loses her husbands (both of them) when she realizes their income can not support her love of lavish living.
So, her husbands (one American, one French) are attracted to her for her beauty. It is the custom of the time that the man work himself almost to death to support the wife's extravagent living habits. In doing so, the couple never really spends any private time together,and when they do, it is not the custom to discuss with the wife the type of work (and in some cases, the legality of it) that he does to support her. Therefore the customs create lonely women, tired men, neglected children, and generally unhappy people.