In order to look for the center of the novel's meaning, look first at the most disturbing elements. There are two young men in competition with one another, and one gives the other one his wife, raising serious questions about his loyalty and sexual orientation. Then, Frankford learns his wife has been sleeping with someone else, another issue of loyalty and sexuality, and he punishes her by severing ties with her. She starves herself to death and dies.
These plot moments have epic proportions, especially in that they all bring permanent, existential changes. For Charles and Francis, their weird competitive friendship has become something like wife-sharing. For Frankford and Anne, the binds of their marital vows were not enough to protect them from Anne's wayward decision-making, and in the end, she literally dies. Frankford is now a widow, and Charles is divorced.
So then, if the novel is arguing anything at all, it seems to be a kind of tragic reminder to be direct in interpersonal relationships. Charles and Francis have some dishonesty about the true nature of their relationship and their competitiveness. Then, Anne's core issue is her disloyalty. If the novel has a thematic message, it seems to be, "Do all your dealings above the table, and keep your word."
