There’s a lot of research that shows animals pick their mating habits based on which is most beneficial for the species. So you generally see most of a species exhibit the same mating habit. Species that have “bonded pairs” generally do so in order for one mate to provide food and protection while the other takes care of the offspring. Species that don’t have bonded pairs generally have either genetic diversity needs or more constrained resources where only the alphas mate (wolves and lions and such)

Humans are more complex animals and have greater ability to think and process emotions. That makes mating take on a whole new complexity.