The endless vacation
The two are at home in their endless travels. Although travel is a sign of luxury, this luxury seems to have rotted; their privileged lives are increasingly lonely and dysfunctional, and although the novel is titled after the symbol of paradise, this story is clearly not about paradise. The irony of paradise and hopelessness is the major struggle of the story. If they always get everything they want, why are they so depressed and hopeless throughout the story?
The affair
Although Catherine urges her husband to participate with Marita in an affair (notice her name is one letter different than "marital"), the affair is plainly dysfunctional. The affair is ironically encouraged, so that the reader begins to learn throughout the story that what Catherine is really doing is controlling an aspect of the marriage she feels insecure about; she is confident that her husband would cheat, so she is proactively making it happen so she doesn't have to experience the pain of paranoia.
Sex and intimacy
The novel proves that although sex is biologically connected to intimacy, there is a counter-intuitive effect in the reality of the situation. Instead of sex bringing intimacy, it removes intimacy from their marriage. It isn't like David is suddenly smitten with Marita. His attention is split between two partners, and the effect is that they are all three perfectly lonely, and their relationships quickly take a turn for the awkward.
The departure
Ironically, Catherine leaves Marita and David alone to go to Paris. The reader can infer that her motives are mixed. Perhaps she is going to punish David for his willingness to sleep with Marita by having an affair of her own. In either case, her departure signals the end of paradise, because she is in need of space and time. She makes excuses for her trip, but David doesn't care anyway; he is too busy with his novel to care about her.
The ironic novel
Although David has two women as his disposal, his first love is clearly his writing. This is an ironic tip-of-the-hat to the novel's author, Hemingway, whose relationships were famously strained in exactly this way. As a writer, David belongs in two worlds; there is the world where he is a participant, the world of the earth and his travels in the Mediterranean, and there is the world of his godhood, because in his novel, he is the master and sole author of the story. The women each ingratiate themselves into his story, calling it "ours," which just proves that they are jealous that his first attention is always to his fiction, and not to their shared reality.
