According to the original paper, 'Postcopulatory sexual selection influences baculum evolution in primates and carnivores' (here), the absence of said bone apparently has something to do with mating habits. The authors suggest there is a relationship between the baculum and the duration of sexual activity. Basically, the shorter the bonk, the less need there is for a bone. They suggest that long sex allows the penetrating male to fend off other suitors while making conception more likely. They surmise that it disappeared in humans because our ancestors started practicing monogamous relationships some 1.9m years ago, therefore our forefathers didn't have to fend off amorous others.
This is a perfect example of ideology masking itself as scientifically informed speculation. It probably wasn't intentional on the part of the authors, but it is worth noting how the rules of evidence and rigor governing scientific study are entirely suspended when one moves into speculation about matters social.
The first point is you can't stick fossilised social relationships from prehistory under the microscope. There is no surviving evidence about the courtship, mating, and familial habits of our ancient ancestors. We can have a guess by having a look at the behaviour of our primate cousins, but as their mating and clan practices show some variation within as well as between species, it doesn't matter how informed the guesswork is.
Second, our scientist friends have come up with an explanation that neglects another defining characteristic of the male member in humans. What we lack in bone we more than make up in length and girth. Yes, it's seldom known but among the primates we are less King Kong, and more King Dong. Gorillas are packing an average 3cm (fully erect), chimps at 8cm, and humans measure up with around 13cm. Why? There are a couple of explanations that sound pretty tendentious. Allow me to indulge some speculation: as the female orgasm is linked to ovulation, and was likely the case in our ancestor species, it is possible evolution selected for bigger penises because sex was more satisfying for our foremothers, who then tended to couple with more endowed males. Therefore men have women to thank for their meat and two veg ... possibly. I haven't a clue, but it sounds at least as plausible as any evolutionary psychologist nonsense. Either way, a convincing explanation of the baculum's disappearance has to address the Big Willy Problem too.
Third, coming back to the monogamy question, our authors suggest we moved to monogamous behaviour to combat the transmission of STIs. This is also unconvincing, seeing as hyper-brainy homo sapiens are still prone to this problem, despite the risks and dangers of disease being well understood. And, again, this is ventured in the complete absence of evidence.
Lastly, when monogamy, or at least the control of women's fertility by male partners did become the dominant reproductive strategy for our species, we're talking 10-11,000 years ago. All the available archaeological evidence points to a coincidence between the development of agriculture, the foundation of permanent households, the production of surpluses over and above the needs of the settled population, the foundation of class societies, and the subordination of women within a sexual division of labour. All of which indicates monogamy became the norm relatively recently in the story of modern humans, and within a blink of an eye if you count our ancestor species. And also, by this time, our baculums had disappeared. In fact, they had vanished completely some 100-200,000 years previously.
This isn't a diatribe against scientific investigation, or a suggestion that oh so wise sociology is king (which, of course, it is). But, again, it's another egregious example of someone in the name of science sallying forth from their discipline and making themselves and those who swallow their speculation look ridiculous. Simultaneously, they're lending scientific credence to and naturalising a set of social relationships that have underpinned the oppression of women as a sex class, and that means their conclusions must be challenged.