The basic premise of this argument, for those unfamiliar, is that monogamy in humans, like the animal kingdom at large, is not "natural." People with Ph.D.'s are getting grants to mythologize about prehistoric human sexual behaviors with reasoning that would be, quite frankly, dismissed as specious and laughable if not stamped with the imprimatur of the university and science. Long ago, in a place far far removed from "civilization," human women copulated freely and often with a variety of men without jealousy or conflict. Sperm competition and not direct or social competition was the paradigm of survival of the fittest. Paternity was never a social question, only one addressed by unconscious drives and natural selection. Then nasty ol' civilization in the form of patriarchy came along and ended this happy time by imposing pair bonding while not holding males accountable for monogamy. Traditional science, a mere outgrowth of patriarchy, then imposed the myth that monogamy was natural and has been with humans throughout our evolution. Only a handful of brave, crusading modern scientists are capable of freeing us from this myth of monogamy and its oppressive presence in our lives.
I honestly have no idea how long ago monogamy was introduced into human history. In fact, I don't even care. It's meaningless, speculative, but worst of all, dangerous, to root around in history and especially prehistory for the "origins" of "human nature." Here's a snapshot of the reasons to reject this kind of mythmaking.
1. Plunger penis and reasoning from morphology.
Where to start... The premise of this argument is that the human penis evolved to have a larger glans to shaft ratio in order to "plunge" semen out of the vaginal canal from previous sexual partners to decrease sperm competition. This kind of mythic retrojection is about as scientific and reliable as Ariel's theories about the surface world based on her conjectures for the uses of the "artifacts" she discovers. Don't get me wrong; I'm a firm believer in evolution. What I don't subscribe to is the notion that the reasonable stories we tell ourselves about why peacocks got crazy tails and elephants have trunks accurately describe the selective processes which allowed those traits to be passed along. Punctuated equilibria and the devastation of a population, such as the theorized Toba catastrophe, create the possibility that certain morphologies didn't contribute directly to competition but were only incidentally inherited. A similar analysis applies to other examples of reasoning from morphology back to our natural state.
2. The origins of modern behavior in prehistory.
Functionally equivalent to the above critique but focused on behavior instead of anatomy, the search for the origins of modern human sexual behavior behind the foggy veil of prehistory isn't particularly helpful. This is true both for those who uphold monogamy as natural and those whose oppose it. One such example, again based on sperm competition theory, is that males who suspect an extra pair mating will, allegedly, initiate sex immediately and thrust deeper and harder in attempts to displace rival male deposits and place his sperm in better position to fertilize the egg. Rape of one's spouse is supposedly reducible in large part to this suspicion. Beyond the questions of (1) is this even true now and (2) how can we know if prehistoric humans actually behaved this way, the latter a wholly untestable hypothesis, this reasoning cannot eliminate socialization as an alternate cause of the behavior. Besides a socialized sperm competition theory, such thrusting may be due to the excitement of the reunion with the partner or have origins in other emotions (as opposed to an inherent, natural human response).
The researcher-storytellers depend on observations of modern "primitive" peoples who are essentially the same, in their view, as our prehistoric ancestors and also essentially "natural" rather than socialized. The reductionist and essentialist premises, as well as problematic methodology, of this assumption require, I think, little explanation. Simply asking why the researcher-storytellers assume these groups to be natural rather collected around cultural forces will collapse the whole house of cards that is their understanding of the unconditioned nature of prehistoric humans.
3. Nature vs. Culture.
Such a heady topic is way too involved to be handled by the informality of a blog. Suffice it to say, these are rough outlines of the debate. Hit me up in the comments if you want pointers towards additional reading.
The myth of monogamy argument is basically that human monogamy is unnatural, it has been imposed upon us by culture. In our natural, prehistoric state, humans did not pair bond. Rather, a female copulated with many males without inciting incidents of jealousy, conflict, or breaking group cohesion. The factors proposed by traditional science for pair bonding, such as female vulnerability while carrying the child, the need for protection, the need for help raising a child, etc. are dismissed as cultural impositions post hoc to justify the unnatural state of monogamy.
The problems with this argument are manifold. First, the very notion of a human nature is debatable, not to mention the problems of actually knowing it AND reconstructing it from before any records were kept. Second, the characterization that non pair bonding in humans, if it was true, was not in fact cultural / social but rather "natural" is questionable at best. Just because it happened prior to the cave paintings at Lascaux doesn't mean it wasn't the product of cultural forces.
4. Immutable human characteristics.
The drive behind the myth of monogamy position, and the one taken up by sex positive activists such as Dan Savage, is that monogamy is not natural for humans. In effect, we are at odds with our very nature when we pair bond. I have more to say about the dangers of "human nature" below, but here I simply want to make the point that even if non pair bonding was a feature of prehistoric human groups doesn't mean this was in fact part of a human "nature" that couldn't change over time.
5. The dangers of "human nature."
The concept of a "human nature" is a dangerous one. What is "natural" and "unnatural" has been the basis of human oppression for centuries. The weaker nature of women was deployed to justify denial of all opportunities beyond the domestic sphere. At the core, the racial sciences used nature to distinguish between the superior (whites) and inferior (Negroids, Mongoloids, etc.) specimens. And the lack of procreative possibility, among others, has long been used in the condemnation of homosexuality as aberrant and unnatural. It wasn't until 1986 that the DSM finally eliminated homosexuality completely from its register of mental illness.
So when somebody like Dan Savage uses "human nature" as a starting point for discussing modern human sexuality, I cringe. Given its historical and continued use for oppressing Others, including gay men such as he, it should be rejected as a liberatory concept. The fight should not be, as he and the many researcher-storytellers currently working in this area contend, about clearing away the untruth from the truth about our prehistoric sexual behaviors, our human nature. Liberation requires us to explode and then abandon the concept of nature itself.
I wish to reiterate. I am a firm believer in evolution. However, I reject the validity of these kinds of studies that seek to draw conclusions about modern human sex from half-baked conjectures about prehistoric human behaviors, modern anatomical structures, and an appeal to human nature and counsel against the dangers of the discursive regimes in which they participate. Liberation won't come from laying siege to and occupying the construct of human nature for the boundaries of the recently seized concept will simply be used to deny others. We must instead continuously break down those walls which delineate the "natural" from "unnatural" to truly be free.