There is a lot of sex in this book, though for the most part it's not overly explicit. There's nothing pornographic here; sex is a part of these people's lives and it's mentioned accordingly throughout the book.
Sex is also one of the novel's most important themes. We see lots of examples of different kinds of sexual relationships, from the trashy to the romantic, and all of them contribute to the book's thematic discussions of sex. Sex here is both an act and a topic for discussion, and it is also a central facet of the book's overall philosophy, which emphasizes individual happiness and desires.
Not to say that sex is overly philosophized here. There are definitely sex scenes that will make you cringe, and ones that will probably elicit a "woo hoo."