I read the book. It was an interesting read. I tend to believe that the human experience is so vast and can vary so much from person to person that by saying anything at all is or isn't "natural" takes away from someone's real lived experience. I think that human beings have a tendency to form connections and attractions to more than one person over the course of their lives. Some people will stay partnered forever with the first person they connected to, others will experience serial monogamy and yet others will be involved with more than one person at a time.
I like that this book starts a discussion about things not having to be the way they are in society today. The more the discussion happens the less there will be a need to hide who you are from the world. I worry about the pendulum swinging too much the other way though and people believing that no one can truly be monogamous and vilifying those who truly feel that they are.
Everything will be ok in the end. If it's not ok it's not the end.