The unsent letters
Letters are symbols that point to communication, so to write them without sending them does something ironic with the epistolary form. First of all, there is no doubt that Chris is still communicating, but not with Dick. She is exploring her own feelings to herself. By writing to a person who never gets the letters, she is able to pretend there is a reality that isn't really there. She can invent Dick in her imagination to be however she wants.
Fantasy and truth
One could say that fantasy points to falsehood, since fantasy is pretending a reality exists that doesn't, but that's not the effect that fantasizing has on Chris. In fact, although she indulges in a secret, make-believe world, she is exploring the truth of her feelings. She doesn't want to be trapped in her marriage; or perhaps she wants the marriage and something else. This becomes clear toward the end when the letters turn to analyze the issue from the lens of feminism.
Feminism
This story does not show a powerful woman manifesting her full potential, and yet the novel points toward feminism. It is precisely because of the effects of patriarchy in her own life that she feels cloistered in her life (especially because of her father's unhealthy idea of masculinity). Yes, she chose her marriage, but she explains that part of the way she was abused in her home was that she was shamed into agreeing with ideas about gender roles that she didn't necessarily believe. Therefore, her desire for Dick can be seen as the desire to break out of the wifely role she accepted upon marriage.
The husband
The center of the novel's use of irony is the husband's tacit acknowledgement and participation in Chris's story. He functions as a male authority and as a wicked judge, because he is at times unsupportive and condescending, or perhaps even patronizing, but also, something can be said for his emotional control. He is ironically composed for someone whose wife is actively seeking an affair.
Dick's response
Dick doesn't get sexual vibes from Chris, even though she writes to him about sexual fantasies she has with him in her mind. His denial and rejection of Chris is an indication that, contrary to her knowledge and expectations, she is coming off more crazy than excited. He is averse to her because she has clearly explored her fantasies about him far more than makes sense, given their limited interactions. Instead of being what she hoped he would be, he makes her question her own motives.
