Some time back, I heard a show on NPR talking about all the recent research on promiscuity in pretty much every species of animals. No mammal or bird (not to mention reptiles or sea life) mate with one other, in a MF configuration, for life.
Yes, there are obvious tribes of great apes, like gorillas, or prides of lions, where it seems one male has several female mates, who are all loyal to him (until another male comes in to openly compete). But in every case of DNA studies of their offspring, there are always those who carry the DNA of other males. The females in every species will sneak off (if necessary) to mate with other males from other tribes/prides. The ones that seem monogamous will become pregnant by other males, but go back to raise the offspring with their life mate, or actual "nesting partner," haha.
Every single kind of animal is not monogamous. Every single kind of animal is promiscuous, even when males try to prevent it.
And then consider the oyster. They all start out as male, switch to female, breeding as both sexes, and sometimes switching back to male again.