Yes, polyandry exists in some species. Saying that only monogamy or only polygyny is "natural" clearly isn't true. But when I include reports of these studies, I caution that what happens in other species doesn't necessarily translate to humans. If three or more humans agree to a polyandrous relationship, they should be free to enjoy it without prosecution, bullying, or discrimination.
This comes from eurekamag.com and is titled "The characteristics and occurrence of cooperative polyandry"...
The relative position of cooperative polyandry among models for the evolution of polyandry and cooperative breeding in birds is discussed. Cooperative polyandry is described as the situation where 1 male and 1 female breed as a group with males sharing equally in copulation and the care of 1 set of young. Sequential and simultaneous polyandry are defined to show how they differ from cooperative polyandry. These systems generally are characterized by the care of only 1 parent for each set of young, a trait which is in sharp contrast to cooperative polyandry. The present models for the evolution of polyandry cannot be expanded to include the cooperatively polyandrous species. The cooperative traits of cooperative polyandry fit within the array of characteristics of cooperative (communal) breeding. General characteristics of all cooperative species (monogamous, promiscuous and polyandrous) are reviewed and possible reasons for the evolution of equal-status males are discussed. The unification of evolutionary models dealing with mating systems and cooperative systems is suggested.
Got that? There are different forms of polyandrous breeding and parenting, and the word "cuckold" wasn't used.