Sexual fetish
The major theme of this novel is the psychoanalytic treatment of sexual fetish. For this, it's important to understand the basic thesis that the characters in the book share: as professional therapists, they all believe that sexuality is at least partially an expression of the subconscious or unconscious parts of the human mind as the mind attempts to cope with trauma or solve problems.
Fetish is what compels Bennett to join his wife in having sex with an attractive man (because he has sexual curiosities about men, and he has sexual curiosities about jealousy). Fetish is what makes Isadora cheat flamboyantly, showing off her indiscretion (the fetish here has to do with shame—she is openly inviting judgment and enjoying that).
Trauma, existential pain, and sexuality
This novel portrays sexuality as a diverse, unpredictable, plastic thing that can be shaped through trauma and experience. The novel shows that a fear of molestation led Isadora into fantasies about being raped that were complete traumatic and horrifying, except that they were helpful to her at a time, because they helped her to cope with memories she could not stomach otherwise.
The novel never draws a moral conclusion from any of this, but the opinion of the psychoanalytic community seems consistent throughout the novel—they basically understand themselves to be compelled to sexual adventures, and they try to learn from them along the way.
Fantasy versus reality
This book is filled with accounts of how sexual fetish shapes a given character's perception of reality. In one of Isadora's stories that she tells her European boyfriend, she describes her relationship to her first husband, Brian. Brian was an undeniable genius, so genius in fact, that he was able to imagine himself into fugue states. Over the course of time, he drove himself mad, until he was having messianic delusions of grandeur, but he was also extremely violent and aggressive—he even raped her.
On the train, she is molested by a stranger. Her husband is also a fantasy-filled person, and he joins his wife (caught in the act of cheating) in a bisexual threesome. These are all ways in which daily life is failing to show the real internal thought life of the characters. How do we know we're supposed to psychoanalyze them this way? They themselves tell us—they're therapists after all.
