Ironically, honesty is something that is only achieved through deception and dissembling. The whole play is built on the appearance that the Hardcastle's house is actually an inn, and that Marlow mistakes Kate, whom he is supposed to be marrying, for a tavern wench. However, through this deception and dissembling, honesty springs forth, as this crucially allows Marlow to declare his honest love for Kate in a way that he would never have been able to if he had met Kate in her role as Miss Hardcastle. Because he becomes so tongue-tied and is unable to express himself well in front of well-to-do women, it is clear that he is a character who needs this deception to let his honesty come forth. Note how this occurs in Act V when he declares his love for Kate:
By heavens, madam, fortune was ever my smallest consideration. Your beauty at first caught my eye; for who could see that without emotion? But every moment that I converse with you, steals in some new grace, heightens the picture, and gives it stronger expression. What at first seemed rustic plainness, now appears refined simplicity.
Part of the humour of this play is the way in which a distinct lack of honesty, in particular expressed through the ways in which the other characters conspire to keep Marlow believing that he is in an inn and that Kate is a serving wench, create the precise situation needed for him to be able to be honest about his feelings. Ironically, in the above quote, Marlow identifies the truth about Kate, as indeed there is more of "refined simplicity" in her character than he would at first believe.