The nature of human nature is the theme of the book, how it evolved, how human sexuality evolved. Reproduction is the sole goal for which human beings are designed. Nothing in our nature has not been carefully chosen. Reproductive success is the examination that all human genes must pass.
Human culture is a product of human nature, not just of our free will or invention. We have laws against racism that allow persons to calculate the consequences of violating them though they may be acting racist because of human nature.
Richard Dawkins argues that the body is merely an evolutionary vehicle for our genes–the genes force the body to do things to perpetuate them. There is selective competition for genes.
People are attracted to people of high reproductive potential, the consequences of which are sexual selection. Even behavior is adapted to this goal. We are descended from men who preferred fertile women. The goal is not just to survive but to breed. The human intellect itself has evolved to contribute to reproductive success.