There are very few rules for the Mating Game. Some species have more than others, and some have next to none. For instance, tropical squirrels are one of the species with very few rules, first to reach the female, wins. Some species of birds will mate with multiple other birds, and often more than once with the same ones.
The mating game has few boundaries of right and wrong. Male lions will sometimes even invade other families of lions and kill many cubs and kick the other male out of his home, and then mate with the female.
Even though the birds and squirrels may have no morals for mating, primates only have one mate. With that one mate they reproduce multiple times, and so do their next generation. Primates are most like humans, not only because of their mating habits, but for nearly everything about them.