It's wrong to lie to the people you sleep with.
Hannah met Bob last night, who is married, and drunkenly slept with him and the next morning she asks him if he's married.
Bob wants to answer Hannah's question.
Bob tells Hannah that he is indeed married and that he has been for a while.
Bob tells Hannah that he is not married and that he actually never has been.
Hannah is glad to know that this was only a one time thing, and she doesn't have to get attached to Bob.
Hannah falsely believes that maybe there could be more to their relationship before she's later heartbroken when she finds out Bob is married.