Oscar Wilde had a gift for writing colorful characters. His characters are witty and clever; they trick each other with lies and bribes, and they always seem to trip over their own egos. Most importantly, they have bold opinions, like each character's opinion of marriage in The Importance of Being Earnest. Most of the main cast sees marriage as a novelty or tool, but Jack sees marriage as a happy commitment between lovers.
In the first act, Jack makes his objective for the play clear in a single line:
I am in love with Gwendolen. I have come up to town expressly to propose to her.
Jack proceeds to comment that he will stop living his double life if Gwendolen marries him, and he scolds Algernon for scoffing at his romance.
Later in the act, Jack proposes to Gwendolen, and at her consent tells her, "My own one, I have never loved any one in the world but you." This certainly lacks the perfunctory tone Algernon takes to marriage, and it is far more intimate than the way Lady Bracknell handles the topic. Jack later tells Lady Bracknell, "I need hardly say I would do anything in the world to ensure Gwendolen’s happiness."
In Oscar Wilde's plays, a character's dialogue cannot always be taken at face value, since characters frequently lie to outwit their scene partners; however, as a general rule of theatre, what a character says in an aside or soliloquy is always true to how he feels. When a character is talking to himself, he has no reason to lie. To that end, here is a line Jack says to himself after seeing Gwendolen off:
There’s a sensible, intellectual girl! the only girl I ever cared for in my life.
This line is convincing evidence that Jack wants to marry Gwendolen because he loves her, and to him, that's what marriage is for.