Jealousy and cheating
The novel includes two separate stories about sexual indiscretion. First, there is the involuntary betrayal that Frankford suffers when his wife sleeps with their guest. Then there is the voluntary abandonment of Susan as a gift to Francis, by Charles. Charles's decision and Frankford's situation are mirror opposites, but both capture a different aspect of romantic jealousy.
Self-starvation as a symbol
When Anne kills herself, she does it by starving herself until her body shuts down. This is symbolic, because she feels guilty for doing the opposite. When she was offered a chance to sleep with Wendoll, she chose to do what she wanted with her body, so now, as a token of her sorrow, she has decided to use her willpower to execute justice against herself (although that is confused and wrong, and no one should ever do that). Nevertheless, the suicide is a symbol for her will to rule over her body, instead of being compelled by her desires.
Dressing his wife up for a friend
Charles helping to dress Susan to look sexy for Francis is a symbolic moment. Although this isn't necessarily the only interpretation, this could be a picture of Charles's romantic love for Francis, where he is using his wife as an avatar to be with Francis. In other words, the willingness to be humiliated this way by Francis seems to be at least in part sexual, or else who would do such a thing?
Nick as a symbol
Nick represents a part of Frankford's own intuition. Notice that Nick is a servant in Frankford's house, an indication that they might be symbolically related. Nick notices something about Wendoll that he despises, and in the fullness of time, it is revealed. Nick intuited that Wendoll was dishonorable, and that he has come with ulterior motives. Perhaps Frankford should have had a higher regard for the part of himself that wondered about Wendoll.
The reverse comedy
The structure of the story is comedy, but backwards. Instead of ending with a wedding and a feast, as traditional comedies do, this novel puts the wedding at the beginning, but by the second scene, there has been a murder, and things only spiral from there. Also, it's interesting that the plot models the first two stories of mankind in the Bible: Adam and Eve and their loss of paradise (Frankford and Anne); Cain and Abel's violent competitiveness (Charles's name even starts with a 'C').
