Marital infidelity and humiliation
There is simply no way to say this tactfully. This book is famous for this theme, so it can't go unsaid either. These stories (along with the rest of French erotic comedy) feature a distinction between "alpha" behavior and "beta" behavior, and the punishment for taking the easy route in life (in these stories at least) is that one's wife cuckolds him by sleeping with more fit mates. It isn't pretty, but that's what these stories are about, and it's absolutely not subtle in any way. These stories are humiliation stories about humiliated men getting humiliated for their foolish and lazy ways. That does seem to be the main cause for this humiliation, by the way—a person's laziness and unwillingness to adventure.
Lies, dishonor, and vengeance
These stories are also shaped by vengeance, like in "The Snow Baby," where the wife blatantly lies about the origin of her child out of wedlock. For this, the husband takes vengeance on her by waiting until it has been long enough that she begins to believe that he must have truly forgiven her, and then he sells her son into slavery and tells her that her "Snow Baby" melted in the heat of Italy's summer sun. It's a cold-blooded story, and there are many like it in the collection.
Honor and courage
This story is the inverse of a hero's myth. It's not about people who go out and adventure. It's not about war-torn, honorable men of power. It's about weaklings, specifically those able-bodied people who fail to transform themselves into honorable, powerful people. The implication is thematic. The book tries to say, "Don't be like these people," where not being like them means choosing intentionally to be honorable, hard-working, and to follow one's calling or destiny in life, instead of becoming complacent and weak.
