A husband and wife are sitting quietly in bed reading when the wife looks over at him and asks the question

A husband and wife are sitting quietly in bed reading when the wife looks over at him and asks the question....
WIFE: "What would you do if I died? Would you get married again?"
HUSBAND: "Definitely not!"
WIFE: "Why not? Don't you like being married?"
HUSBAND: "Of course I do."
WIFE: "Then why wouldn't you remarry?"
HUSBAND: "Okay, okay, I'd get married again."
WIFE: "You would?" (with a hurt look)
HUSBAND: (makes audible groan)
WIFE: "Would you live in our house?"
HUSBAND: "Sure, it's a great house."
WIFE: "Would you take her golfing with you?
HUSBAND: "Yes, why not?"
WIFE: "Would she use my clubs?
HUSBAND: "No, she's left-handed."

The humor in this joke is two fold: first the audience realizes that the husband has a specific person in mind, which was (up until that point) not expected. The second reason why this is funny is that the husband accidentally revealed this to his wife (which is also unexpected).

Basically this joke is making fun of the husband for being stupid.