Many on the left often advocate the rejection of Christian sexual ethics with the argument that lifelong monogamy is unnatural. And by unnatural they mean not merely among human beings, but among animals. In their worldview, of course, man is simply an animal—a highly evolved animal, to be sure, but an animal nonetheless. And how better to gain an understanding of ourselves than to study what is “natural” in the animal world? David Barash, author of The Myth of Monogamy, tells us: “There has been quite a revolution in scientific understanding of the lives of animals and we can learn a lot about ourselves by looking at other creatures.” Presumably, Barash would take exception to Pope’s famous line, “The proper study of Mankind is Man.” Perhaps he would wish to rewrite it to something like, “The proper study of Mankind is Manimal.”
Meghan Laslocky, author of The Little Book of Heartbreak: Love Gone Wrong Through the Ages, opined for CNN, “The bottom line is that flings are far from folly, at least in the animal kingdom.” Her piece was accompanied by a slideshow telling us such helpful things as: Penguins mate for a year and then move on to a new partner; male elephant seals have harems of as many as 100 females; Bonobos regularly engage in frequent sex with multiple partners; swans, which have been traditional symbols of fidelity, really aren’t monogamous.
At the Huffington Post we learn what can only be regarded (by some at least) as a startling statistic: “Only 3% to 5% of all the mammal species on earth practice monogamy.”
In all these observations there is an implicit argument that runs something like this:
What is natural for animals is natural for human beings.
Having multiple sex partners is natural for animals.
Therefore, having multiple sex partners is natural for human beings.
There is another argument implied, too, one that uses the conclusion as the minor premise.
Everything natural is good.
Having multiple sex partners is natural.
Therefore, having multiple sex partners is good.
Looking to the animal kingdom to find norms for human behavior, however, is an instance of what Paul refers to as worshiping and serving the creature rather than the Creator (Rom. 1:25). As Rushdoony has pointed out, “in any culture the source of law is the god of that society.” The same can be said of ethics, for law is simply the institutionalization of ethics for application to society. Looking to the animal world for guidance in ethics is, in effect, the divinization of the animal world.
Animal-based ethics is an inversion of the created order. The pre-fall order was:
The fall involved man paying heed to an animal and asserting a right to overrule God so that the order was inverted.
This is essentially the order which is now being advocated. But are we really sure we want to travel this road? I happened across this article yesterday, Chimps are naturally violent, study suggests. As it turns out, chimps will attack other “communities” of chimps in order to increase the size of their territory, gain access to greater supplies of food, and have more females with which to mate.
For years, anthropologists have watched wild chimpanzees “go ape” and attack each other in coordinated assaults. But until now, scientists were unsure whether interactions with humans had brought on this violent behavior or if it was part of the apes’ basic nature.
A new, 54-year study suggests this coordinated aggression is innate to chimpanzees, and is not linked to human interference.
“Violence is a natural part of life for chimpanzees," Michael Wilson, the study's lead researcher and an associate professor of anthropology at the University of Minnesota in Minneapolis, told Live Science in an email.
You can watch an example of this aggression, including chimpanzee cannibalism, here.
Since this is “natural” is it also good? Should it be used as a guide for human behavior?
The evolutionary presuppositions of those who advocate animal-based ethics deny the most important thing to know about man, namely that he is created in the image of God, which means that he is qualitatively different from—and superior to—the animal kingdom (Gen. 1:26-28). Scripture admonishes us, “Be not like a horse or a mule, without understanding” (Ps. 32:9). Peter echoes this when he describes certain men who “count it a pleasure to revel in the daytime” and who “have eyes full of adultery” as being “like irrational animals, creatures of instinct” (2 Pet. 2:12).
Animals are irrational. They are governed by their instincts, their appetites, their urges. But as bearers of the image of God, we are called to better things.
 Rousas John Rushdoony, Institutes of Biblical Law, p. 4 (emphasis in the original)
 I am indebted to Steve Schlissel for this insight.