What does The Importance of Being Earnest tell us to think about marriage? What does it mean that marriage is such a subject of derision in the beginning of the play, and yet everyone gets married in the end?

The play communicates, ultimately, the idea that marriage is a necessary element of a functioning society, even if it is uncomfortable or ridiculous. The fact that all of the major characters, even including Miss Prism and Doctor Chasuble, end up married in the end demonstrates the inevitability of one's eventually "settling down." Algernon may be used to scampering around London, doing whatever he pleases—including inventing a fictional, invalid friend named Bunbury who is perpetually at death's door, especially when Algy wants to escape a social obligation—but he eventually settles down with Cecily, with her fine profile and large fortune. Jack is, likewise, a total liar, as he invents a fictional brother whose identity he can take on whenever he goes to the city so that he can escape the confines of the strict moral code he's adopted as Cecily's guardian. Yet, he also finds love and marries Gwendolyn. If even these two dissipated scamps eventually give in to the marriage state, there can be little hope for someone more conventional.