As one who lived and wrote during and after the horrors of World War I, Noel Coward was influenced by artists who became expatriates after the war in an attempt to escape from the deterioration of society. Officially referred to as the Lost Generation, writers like Gertrude Stein and F. Scott Fitzgerald left their home countries to live in France and other places because they perceived their old, socially constraining ways of life to be superficial and materialistic, and they instead embraced lives of immorality and recklessness (Encyclopaedia Britannica, "Lost Generation"). Coward's play Private Lives satirizes their escapism and lack of morals through his characters.
The characters Amanda and Elyot embody the expatriates in France whom Coward was friends with. Their upscale life in France is completely devoid of any social and moral constraints. They married each other without a true understanding of what it meant to be married, and because they had no social and moral constraints, they very quickly filled their married life full of violent arguments and jealousies. After their divorce and after they remarried and saw each other again, their lack of moral constraints quickly led them to abandon their new spouses and run off with each other. Amanda underscores the theme of lack of social and moral constraints when, in a discussion with Victor, she says, "I think very few people are completely normal really, deep down in their private lives" (Act One). She further adds, "There's no knowing what one mightn't do" and describes she and Elyot as "two violent acids bubbling about in a nasty little matrimonial bottle" (Act One).
In contrast to the socially and morally free-floating Amanda and Elyot, Sibyl and Victor represent classic social constraints and expectations. They embody the expectations of marriage by wanting to take care of their spouses. Victor is a classic, strong gentleman, whereas Sibyl is a classic romantic. Yet, even this adherence to social constraints do not make the characters happy.
Hence, Coward is showing us that there must be a balance between the two extremes. While it is wrong to be so without bounds that you are self-serving and violent, it is also equally wrong to be so constrained by society that a person does not develop into the person he/she could truly be.