Satire in The Importance of Being Earnest
Gwendolen all but calls Cecily provincial, implying that by knowing of and casually referring to a gardening tool like a spade she's revealing herself to be of a lower class. Of course, Cecily has merely used the well-known phrase, "Call a spade a spade," which Gwendolen has no doubt heard before and understood perfectly well. In her deliberate refusal to recognize this idiom, the audience can see the difference between Gwendolyn's satirical one-liners, which come from a place of cruelty, and Algernon's witticisms, which seem merely to be in jest.
Of all the themes in the play, perhaps the most important one is that of "manners" or propriety. This idea of how one most behave in polite society pervades throughout Act I and II, affecting the way the main characters think, speak, and interact with each other. In fact, most of the comedy in the play stems from Wilde's satire of these "manners" and their inherent strangeness, which Wilde characterizes here as both shallow and performative, a kind of "mask" that people wear in order to get by in the world.
Gwendolen inverts traditional gender roles, saying that men are best suited for the home (a sphere typically reserved for women). In doing so, Gwendolen asserts her power as a strong young woman, while at the same time subverting gender stereotypes and putting Algernon in his place along with her father. As with all the subversive ideas in the play, Wilde uses this inversion of gender roles for comedic effect, but the audience can assume, given his satirical tone, that his critique of modern British society still stands.
Lady Bracknell summarily lumps marriage in with natural events like the weather and indigestion, suggesting that the social construct of marriage (which has, in its way, dictated all of Jack's and Algernon's actions) is in her opinion no better than a mere physical occurrence, and that it meant absolutely nothing to the General, as she suspects it will mean nothing to Jack and Algernon as they age.