The study of sexual physiology what happens, and why, and how to make it happen better has been a paying career or a diverting sideline for scientists as far-ranging as Leonardo da Vinci and James Watson. The research has taken place behind the closed doors of laboratories, brothels, MRI centers, pig farms, sex-toy R&D labs, and Alfred Kinsey's attic. Mary Roach, the funniest science writer in the country (Burkhard Bilger of The New Yorker), devoted...