How are love and marriage treated in The Importance of Being Earnest?
As the complete title of the play specifies, The Importance of Being Earnest is "A trivia comedy for serious people." This means that those themes which are universally considered, or treated, in a "serious" way, namely, love and marriage, will also be treated trivially, or with little importance, throughout the play.
While the ultimate goal of the play is for Jack (Ernest) Worthing to marry Gwendolen, and for Algernon to marry Cecily, the motivations and means by which these goals are attempted are as ridiculous as they are comical.
Jack says that he loves Gwendolen. His passion is supposedly reciprocated, but the relationship cannot be possible unless Jack shows Gwendolen's mother what sort of family he comes from and who are his relatives. Lady Bracknell, an arrogant aristocrat, just cannot tolerate Jack's life story that he was found in a handbag at a cloakroom in Victoria station, "the Brighton line." Hence, in order for a marriage to be possible, Jack needs to "produce" a father and a mother (a family name) in short notice.
What this shows is the shallowness of the entire thing. Marriage is seen as a transaction of family names and fortune, and not as a love connection. Gwendolen is all too familiar with this, as even she says that
...although (Lady Bracknell) may prevent us from becoming man and wife, and I may marry some one else, and marry often, nothing that she can possibly do can alter my eternal devotion to you
Love and devotion, as well as marriage and loyalty, are flimsy and ephemeral in the eyes of Gwendolen and her mother. Algernon, who is Lady Bracknell's nephew and Gwendolen's cousin, has a similar opinion on the matter.
Algernon: I really don't see anything romantic in proposing. It is very romantic to be in love. But there is nothing romantic about a definite proposal. Why, one may be accepted. One usually is, I believe. Then the excitement is all over. The very essence of romance is uncertainty.
Add to this that the primary motivator of the devotion of the two female leads, Gwendolen and Cecily, is the simple fact that their object of affection goes by the name of "Ernest." Gwendolen says that there is something in the name gives her vibrations. This is primarily why Jack adopts the name and goes by it. Cecily says that the name inspires confidence- which is why Algernon pretends to be the bad, fake brother Ernest (a character invented by Jack to leave the country side with an excuse) and introduces himself like that to Cecily. The name is a lie that will later become an ironic, partial truth when Jack discovers that this father's name was,and hence his namesake would be, Ernest.
The motivations behind the affections of the characters are shallow. The process of the marriage proposal is transactional and matter-of-fact. There is very little space for true love, commitment, and passion. In true Wilde fashion, these facts will be further twisted as a way to take digs at the prudish and hypocritical Victorian society that harbored these types of dynamics. This is also the primary reason why Wilde will choose those very two topics as sources of comical triviality.