WHY IS SEX FUN?: The Evolution of Human Sexuality

To us humans, the sex lives of animals seem weird. But it’s our own sex lives that are truly bizarre. We are the only social species to insist on carrying out sex privately. Stranger yet, we have sex at any time, even during periods of infertility, such as pregnancy or post-menopause. A human female doesn’t know her precise time of fertility and certainly doesn’t advertise it to human males by the striking color changes, smells, and sounds used by other female mammals.
Why do we differ so radically in these and other important aspects of our sexuality from our closest ancestor, the apes? Why does the human female, virtually alone among mammals, go through menopause? Why does the human male stand out as one of the few mammals to stay with the female he impregnates, to help raise the children that he sired? Why is the human penis so unnecessarily large?