Kate wants a man to marry her for love, not because of her social position. By pretending to be a barmaid, she can get to know Marlowe's true character. Barmaids and lower level women were the object of sexual harassment quite often. Marriage-especially in this story- was a formal, business-like arrangement, not a romantic connection. Kate does not want someone to marry her for this reason.
Marlowe has an attraction to "barmaid" Kate. They enter into a smitten friendship. Eventually, Marlow realizes that he loves her, and wants to marry her despite her social status. He knows that his family will not allow it.
In the end, Kate constructs a way for people to see that Marlow truly loves her-not her appropriate social status.
If Kate had not stooped to deception, she would always have doubts about the sincerity of her husband. Social restrictions made it impossible for two people of a certain class to form an intimate relationship. By pretending to be of a lower social class allowed her to escape this restriction.
In many ways, Goldsmith's She Stoops to Conquer satirizes the ways the eighteenth-century society believed that proper men and women ought to behave. While the play shows the traditional pattern of male-female relations in Hastings's wooing of Constance, it also reverses the era's sexual etiquette by having Kate pursue Marlow. Kate is fighting against the idea that women are commodities to be passed along through marriage, and asserting her power and independence by choosing and wooing a husband herself.