One couple wants to overcome their sterility, another couple wants to stop having babies, another to have a girl next time. Such problems are thought to be medical ones, not sociological ones. Dogs have puppies because they like to have sex. Goats have kids because they like to have sex. But I think this answer is fundamentally wrong when applied to people.
Humans can fulfill a sex drive without it leading to reproduction.
It certainly does not have an easy answer. In truth fertility, in every society, always has been socially controlled. There is no simple instinctive explanation for why humans have the number of babies they have. It certainly is true that we have a sex drive. The anthropological and historical evidence is overwhelming on this point.