Other then the reason for having children, people have sex for the pleasure it creates. Some people use sex as a stress reliever, and a select # of people just do it to do it.
people have different reason why they do s ex some is to have offspring some because they love each other some just because of needs and because the pleasure that sex can give to them
your answer is probably the best one so far. i'd like to add that some people do it because they're addicted to sex also.