And someone came along, and beat them with a clue-bat. Not that it'll to much good for the Lefties. What you have to realize is that this wasn't some Lefty trying to figure out why we are the way we are. This was a Lefty who wanted to normalize his lifestyle (or more likely, his desired lifestyle), and thus created bullshit out of thin air and put it to a video. For example:
Funnily enough, in the source from which Vox sources its testicle size data, the volume Sexual Selection and the Origins of Human Mating Systems (2009), primatologist Alan F. Dixson argues against the ideas Ryan describes regarding sperm competition in humans.
So the Lefty quotes a source to support his claim even though his source directly refutes his claim. This is classic Leftism. Because the Lefty didn't think anyone would actually look at his source. He thought that all his fellow polygamists would nod their heads, say "Oh he's so smart!" and then his video would be used over and over and over until it became canon law.
Rather than explaining monogamy, the Vox video portrays it as something alien and largely useless. It seems like this particular narrative was deliberately chosen before a frame of video had been shot. The expert they interviewed who got much of the detail right was David Barash, yet he was given the least screen time. I suspect they chose him because he wrote a book (which the makers of the video almost certainly did not read) called The Myth of Monogamy. But once they started talking to him they were perhaps disappointed to discover that the title refers to the historical prevalence of polygyny, and not the polyamorous utopia described by Christopher Ryan.
Emphasis mine. Welcome to the world we live in.