This is a book about sex in long term relationships. Perel is a therapist who sounds a little like the narrator of Sex and the City the book. Bushnell talks to a lot of married couples, mostly in an attempt to report on issues based on the people she knows. Perel unearths similar storylines from her patients in order to resolve them and also to understand them against the backdrop of American culture.
Perel uses physical boundaries to understand relationships. She says that pairing up with someone still requires there to be two separate individuals to maintain eroticism. If you add a kid to that eventually, often the group splits off another pair (mom and baby). Now the dad is the third person, and he needs to re-romance in order to be her erotic partner again. The existence of other people (whether in fantasies or in real life) are another kind of third person, always present in long term relationships.
Her solutions usually require people to closeread their fantasies and to keep dating the person after you’re together.