Females can reproduce only once every 9 months. It's an investment and, therefore, evolutionary advantageous to be picky. Males can reproduce every four hours, give or take. It's a quantity game. This promotes competition, which helps females assess better potential partners and, in turn, makes them compete among themselves to secure superior partners. It is, however, also evolutionary advantageous (for both males amd females) to protect your progeny. This is why humans are typically considered socially monogamous (typically having one primary mating partner but potentially having flings on the side).