What is the reaction of Morell when Marchbanks declares that he is in love with Candida?

At the end of Act I, Reverend Morell is bragging about his and Candida’s happy marriage to the young poet Marchbanks, who is unmarried. The younger man challenges this notion and insists on settling something between them: “I love your wife,” he declares. George Bernard Shaw’s stage directions state that

Morell recoils, and, after staring at him for a moment in utter amazement, bursts into uncontrollable laughter.

Continuing to laugh, he sits down and speaks condescendingly to Marchbanks, noting that “everybody loves her.” Given the ten-year age difference, he dismisses it as “calf love” (like “puppy love”) and criticizes Marchbanks’s foolishness. The younger man calls Morell out on his “complacent superiority,” claiming that Morell is willfully unaware of the passions that Candida can inspire and rejecting Morell’s claim that his love is not real or meaningful. “Do you think that the things people make fools of themselves about are any less real and true than the things they behave sensibly about?”
Morell continues to lecture him about the genuine love he will find later in life and how maturity will bring understanding, until Marchbanks rebukes him for relying on oratory instead of speaking from his heart. It briefly seems that the men will fight physically, but they continue to argue until Marchbanks declares that he will “rescue her from her slavery” to her husband’s “clergyman’s ideas.”
After Morell utterly rejects Marchbanks’s demand that he tell Candida of their conversation, Marchbanks further declares that he will write to her, and criticizes the husband for indulging in a lie. Candida then enters the room, and Marchbanks leaves the house. The situation is finally resolved at the play's end, after Candida reveals to Morell that she is aware of Marchbanks’s feelings for her.

Candida is a comedic play written by famous playwright George Bernard Shaw, which was first published in the year 1898.
Eugene Marchbanks is a socially awkward eighteen-year-old poet and nobleman who falls in love with the beautiful Candida. Unfortunately for him, Candida is already married to Reverend James Morell, who is a Christian Socialist clergyman. Morell and Candida take Marchbanks under their wing after Morell found the young man "sleeping on the Embankment." The married couple clearly love each other, but Marchbanks convinces himself that Candida dislikes her husband and their marriage and would choose him over Morell if given the chance.
At first, when Marchbanks declares his love for Candida, Morell laughs and says that everybody loves his wife and that Marchbanks' case is nothing special. As Marchbanks goes on to insist that Candida secretly despises her husband, Morell takes his insulting outbursts more seriously and begins to get angry.