It's not just the drive to reproduce. Sexual contact is a powerful force. It binds relationships, families, communities, cultures, even nations together in ways we seldom even consider. It is part of the glue that holds people to people, that brings us together. It is one of our greatest gifts.
Sex is, for human beings, a vehicle for contact and communication on the most profound of levels.
Sex is any contact between people that is erotic in nature and which potentially can lead to orgasm. This is an enormous range of contacts, from sexy phone calls at one end of the spectrum to copulation for intentional pregnancy at the other. Some of them can be done casually, some require a good deal of forethought, preparation, and responsible behavior. The important part is knowing which is which. In both cases, only the exchange of body fluids will cause the aforementioned problems.