There is a very interesting discussion about that in "Sex at dawn". Shortly put, anthropologists are very biased, and they will use their data selectively.
Today, there are people that live wholesome lives with multiple consensual sexual partners, search for polyamory.
IMHO, there simply is a lot of residual prejudice coming down from old religious dogma.
It wouldn't at all surprise me if human sexuality adapted flexibly to the social environment. Both can be true. Marriage might be a natural and ubiquitous response to the pressures of living in settled relatively highly communities, while flexible relationships might be common in societies of small loosely associated nomadic groups.
Show me evidence of this. Blaming all the anthropologists for being prudes and suppressing the data that everybody wants to hear and that would make them famous is just not even remotely believable.
I suspect that it was the advent of agriculture that led to monogamy as we know it. Now that almost nobody works in agriculture any more we are reverting to the relationship styles we evolved with.
Prejudice is inertia. It muffles your decisions. It can be stabilizing and it can drive you off a cliff even though you're flooring the breaks. A truly stable system is flexible and always has inertia under control.
There is a very interesting discussion about that in "Sex at dawn". Shortly put, anthropologists are very biased, and they will use their data selectively.
Today, there are people that live wholesome lives with multiple consensual sexual partners, search for polyamory.
IMHO, there simply is a lot of residual prejudice coming down from old religious dogma.