At least for humans, this most basic of acts is anything but basic. As the pioneering sex researcher Alfred Kinsey put it, the only universal in human sexuality is variability itself. From attraction to action, sexual behavior takes many forms. Human interest in sex is not a matter left to chance but more a built-in imperative; survival of the species depends on it. The decks are stacked in sex's favor, as a passport to bonding, to intimacy, to pleasure, and even to human growth and healing. Bodies and interests change over the course of time, and the complexities of physiology and psychology mean that most people experience a sexual problem at some point in their lives. Although sex can be one of the most difficult topics for partners to discuss, it's one that also stands to draw couples closer together. The moral and political implications of sex vary greatly from culture to culture, and even within cultures and over time; still, there is agreement on one certainty: It's why we're alive today and what future generations depend on.