When children ask why people have sex, they often hear about love and expanding on that love by starting a family. Those reasons for sex are pretty accurate, although they leave out some of the finer points. With a more mature viewpoint, it's worthwhile to consider the finer points about why people have sex.
Making babies: Biologically, people are hard-wired to have sex in order to keep the human race going. In other words, at its heart, sex is about making babies — generally and historically speaking. With the advent of contraception, sex and pregnancy aren't inextricably linked.
If you absolutely, positively don't want to make a baby, then don't have sexual intercourse — be abstinent. No birth control is foolproof.
Enjoying a sensory experience: Many people engage in sex for the sensory experience, the wide range of physical and emotional pleasures that a person can derive from sexual activity.
Partnering up: Sex can play a role in forming a bond with another person, which is yet another need that humans have.
For love and the long haul: The drive to find someone to love remains a strong one. Part of the glue that holds longtime love together is sex.