"Second Sex" is one of my favorite books to teach, and De Beauvoir is an incredible writer and thinker. However, this idea of the 'primitive' is a major problem in sociology, anthropology, and other disciplines at that time period. This is a huge issue with Durkheim as well. Even though his "The Elementary Forms of the Religious Life" is still a classic, and useful text. It must be pointed out when using it that his idea that Australian Aboriginals represented a 'primitive' society and where therefor closer to the source of the formation of religious thoughts is deeply flawed. I don't wish to make a 'woman of her times' argument for De Beauvoir- partly out of respect to her substantial intellect, and partly because she could have done better. However, it was not out of step with the-now very dated- thinking in the social sciences at the time.

As has been pointed out across the years- see Graeber's "Toward an Anthropological Theory of Value" for example- a tribal society that exists in the modern era is not 'primitive', they exist in the modern era. They are effected by the societies that surround them. They also grow and change based on their own internal tensions, conflicts, and successes. We can observe some tribal societies in certain conditions and try and understand how people in the past might have behaved, but these societies are not simple proxies for ancient life.

So, in short, yes, it is a big problem, but not isolated to De Beauvoir.

This is something that's done frequently in archaeology (particularly by 'New Archaeology ' of the 1960s) and is still quite frequently done for Neolithic and Bronze Age cultures. Sometimes the comparisons are quite valuable for thinking about how past societies may have functioned or done something, but more typically (in my opinion as an EBA archaeologist) they're cherry-picked anecdotes that can easily be refuted by any other cherry-picked ethnographic anecdote. There's a really good example of this from the Neolithic site, Göbekli Tepe where the excavators make claims about how the site functioned and how long it took and the extreme effort it took to carve and move the stone pillars into place.

On one hand, the authors use populations in New Guinea to exemplify the existence of sacred buildings in hunter-gather populations and the intertwined nature of ritual and everyday life. However, on the other, they insist that there were no domestic or village components of Gobekli Tepe. This seems like a strange comparison to make, where they seemingly pick-and-choose which aspects of the ethnographic evidence to apply. Another note is that New Guinea is the second largest island in the world and contains dozens, if not hundreds, of different tribal groups, making labelling the ethnographic data as simply ‘New Guinea’ rather vague.. (see Dietrich & Notroff 2015, link)

The authors also assert that BanningBanning 2011 denies the high effort required in erecting the enclosures, however Banning actually points out that while he recognises the impressive effort that must have gone into constructing the site, the experiment on Easter Island used by them was incorrectly described. In fact, the study found very few people were required to move and erect a stone with a mass of 10 t. This is a contrast to the ‘hundreds’ needed as postulated by Notroff 2014 paper.

The difference in their estimates was pretty significant, and highlights the caution of using ethnography and modern anthropology to reconstruct prehistoric societies. I'm sure there are many better examples out there (a lot of "X" culture has a highly stratified gender separation and therefore 6000 years ago here on the other side of the world they did too), but the Göbekli Tepe always stuck out to me because it became a dialogue between two archaeologists who often published arguing about it and the site's function over several years.

Intentionally not addressing the use of "primitive" here as it was ready discussed in an earlier comment. So is it "wrong" to use ethnographic sources? It depends. Morally, I think it depends on how you use it and why. If you lean into the "primitive" notion, then yes. The other major issue is as I've tried to highlight, that generally ethnographic sources are misused and often pretty irrelevant anecdotal cherry-picked ideas to relay the author's own ideas.

The use of modern observations to understand past peoples is broadly called "ethnographic analogy." There are entire books on how to do it, when to do it, and why you should never do it.

The short answer to your specific question is that ethnographic analogy can never be used as proof for a claim, for reasons that seem obvious to you. One group of people doing something is not evidence another group did it. This doesn't apply to beliefs of people in contact with each other in nominally the same religion, so it certainly can't apply to very specific gender concepts across millenia.

There are certain broad claims that can be ethnographically analogued, and it's clearly not productive to look at every new archaeological site as entirely its own thing. I can anticipate certain buildings in the Roman village I'm excavating because it's a Roman village, and a mesolithic camp that appears to have been inhabited by a foraging community will presumably contain burnt, discarded animal bones and other signs of hunted game. These aren't much of a jump, however, because the claims we are making are derived from the same societal traits by which we grouped the people.

That is, if I have grouped two communities by their subsistence strategies (foraging, horticulture, agriculture, etc.) I can make reasonable assumptions about practices related to subsistence. If I have grouped two communities by social identity they claim to share, then I can make reasonable assumptions about the things that identity entails. 

Problems come when we try to move beyond that.

De Beauvoir was writing in a time when we new terribly little about the neolithic, and what we did know was largely based in a rudimentary model that saw early peoples defined above all by their subsistence strategies and tool-making technologies. As I will get to later, this was partially due to the infancy of the field and the limits of existing data, and partially due to a hesitancy to make larger claims. Rregardless, this led to a lot of statements by authors in many fields that were variations on "Because they were farmers, they did this..." or " Because they were hunter-gatherers, they did this..." Again, that's fine if you are going to make claims about the farming they were doing.  But when you jump from subsistence strategies to big cultural concepts like gender, you're moving beyond what the data can tell us.

What does the data tell us? Well, absolutely nothing. It is nigh impossible to extrapolate with such detail what the first agriculturalists thought of gender. Could there have been associations between female fertility and the fertility of crops? Certainly. That's hardly a novel claim since it's basically the same thing. But the ancient cultures I study also associate the severed trophy heads of sacrifical victims with the fertility of crops, and human fertility is so tied to ancestor veneration that multiple artistic tradition depict women sexually stimulating skeletons or mummies. And I don't see de Beauvoir talking about that.

But we shouldn't blame de Beauvoir here. Early archaeologists tended to over emphasize the significance of key milestones that made us who we are today (read: that made European civilization what it was in 1850). They used these milestones to delineate stages of sociopolitical evolution (savagery-barbarism-civilization or band-tribe-chiefdom-state), which effectively packaged a bunch of technological and cultural things together- you've got sedentary villages, so you've got agriculture; you've got writing, so you've abandoned animism. Others have expanded on this in the thread. What's important is that the past decades of research have given us greater resolution for the timing of important "first" and of the chronology of specific sites. These data tell us that things like agriculture or sedentary cities were slow, gradual developments that were experimented with, given up on, and reinvented many, many times in many, many places.  Regardless of what de Beauvoir claims, the general idea that we can make any such statement about he cosmology of the first farmers hasn't been accepted for some time because “the first farmers” encompasses so many diverse peoples.

I crossposted this to r/AskAnthropology and

u/Vo_Sirisov gave this answer:

I would say it is not appropriate in this instance, and that de Bouvier's writings are a relic of a very different approach and mindset from what we strive for today.

Unfortunately even today though, there is an unfortunate tendency for laypeople and even some anthropologists to perceive so-called "primitive" hunter-gatherer cultures, such as the Sentinelese or indigenous Australians, as having being frozen in time compared to the rest of us.

In truth, this is obviously not the case. These groups have gone through just as much cultural evolution as the rest of us, just along a different path.

Studying the lifestyles of such groups can allow us to learn broad strokes about how hunter-gatherers live, but it cannot with any reliability tell us specifics about the culture of our mutual ancestors.

u/mcotter12 gave this answer:

You are right, it is. And the answer is yes.

They did this with a tribe in the Amazon in the 70s or 80s. They called them the most primitive people in the world. It turned out they'd been living outside a suburb of Rio de Janeiro until a generation ago and being displaced destroyed their way of life completely

EDIT added answer, formatting

I think now people and historians think more like you describe here. They understand that every culture and society has 'developed' just as much as any other one. They have simply done so in response to their own conditions and crises putting them on vastly different trajectories than those societies generally obsessed with scientific advancement, written history and sociological categorization. That being said, I do think historians currently do still look at tribes and cultures that function under similar conditions to what we've deduced other ancient cultures must have inhabited to find similarities. A current example is the Hadza people of West Africa who still operate as hunter-gatherers. However, their own oral history has something of a record of their own technological advancements. So, everyone knows their current way of life is different from that of their historical ancestors. There is another lesson that can be learned these people. As there exists ancient rock art in their territory that was created thousands of years ago, and the Hadza claim that art was made by their ancestors, but the modern Hadza do not create rock art. Does that mean the Hadza histories are wrong about who made it? Maybe, but more specifically, it is a reminder that only those things actively practiced and taught in the culture survive to future generations. We don't often get to see evidence of things a culture discarded along the way to its current state.

That is kind of a huge gap in our attempts to piece together anything from ancient societies. The Australian Aboriginal culture is probably our best resource to know something about truly ancient people as we've been able to confidently say aspects of their culture date back over ten thousand years because their oral histories match the geological record. This seems like an amazing glimpse into seeing something like a preserved fossil of human culture, but they too are a current culture. That they have been able to preserve some ancient customs and tales is remarkable, but it is still limited. We can never really know what things the culture didn't choose to preserve. What isn't deemed important is forgotten. So, ancient techniques of tool making or art-making or storytelling could have been lost and rediscovered a hunded times over in one culture but never appear in current practice if better methods were developed to replace them. We only ever get to see what people actively preserve through use, practice and education. We also only get lucky every once in a while to be able to confirm a culture's historical accuracy with artifacts or geological evidence.

In summary, historians do still use some current societies to try and ascertain how ancient social groups may have operated under similar conditions. However, even those groups have histories or myths of their own technological development. Meaning they themselves know they live different lifestyles from their own ancestors. Also, though long-surviving cultures may retain knowledge or stories directly from ancient times, we can never know those things a culture failed to preserve about itself. We may be able to find evidence of those lost things, but if their methods of creation or purpose were discarded by their culture at some point then we are out of luck. Essentially, even when historians can use some current societies or cultures to model how ancient humans might have lived, we still know that we are getting a very incomplete, skewed and edited model about what that might have been. However, we are lucky enough to live at a time when we have been able to find and identify people and cultures that have successfully preserved at least some knowledge that is far older than any written record.

You've already gotten a lot of fantastic answers. I just wanted to throw in a previous discussion of mine on the topic of whether there is a universal correlation between patriarchy and prehistoric agriculture here, which is a claim based on "analogies" like the ones you describe.