A just-so story is unverifiable.
You can verify that reproduction is physiologically directly more costly to female humans than to male humans, and it is logical that natural selection would select those that behave as though it were true over those who behave as though it were not true.
For nature to select those who behave as though this weren’t true, as much as those who behave as though it was true, there would have to be no reproductive consequences for incurring more cost from reproducing, which requires a transcendence of nature that we’re only part way done with achieving.
At the very least, females are born with finite eggs and pregnancy takes a longer amount of time and uses up more resources than a male has to deal with. At the most basic quantifiable level, it would be on average far more possible for a male to reproduce many more times over than the female. This objectively makes an r-species strategy far more viable for a male than a female. Obviously this doesn’t mean that r-species strategies are always best for males (or even females), as evidenced by the variety of strategies that occur in nature and even that you rightly point out occur amongst humans. By your own logic, I don’t think it’s reasonable to force all the alternative strategies that humans adopt into the k-species model. There are plenty of species that fit far more into the pair-bonding model, with low sexual dimorphism, than humans - and plenty that fit far more into the tournament species model, with high sexual dimorphism. Humans on average probably tend more towards the k-species extreme than the r-species, but they are in between - and as you say adopt either, and sometimes phase from one to another over the course of their lifetime.
Tallness empirically emerges cross-culturally as a signal of male sexual desirability. This is far more than Ecmandu’s other two “objectifications” of money and large penis, which are more concerns of insecurity than fact. Penis size isn’t a sexual signal as it’s not part of the sexual displays before the act has already begun - at most it might have some effect on preventing female promiscuity after the initial act. Money is also limited in its effect on perceived sexual desirability in males, as its more of a symptom of what’s desirable rather than desirable in itself. Women don’t just want money from a male, they want good genetics to pass onto their offspring and a reliable signal that money now will still be money later. Tallness appears to be a far more reliable signal to this end, but whether or not this is part of why tallness is cross-culturally sexually desirable in males, the fact remains that women want a male who is at least taller than them: the definition of sexual dimorphism, even if other species are more extreme in this sense. It’s still true that sexual dimorphism in humans isn’t that pronounced, but it’s definitely there.
I didn’t mean to imply that this means there’s only the one narrow strategy that this most simplistically leads to for males and females respectively, only that the mean tends towards what Ecmandu was saying about his “Three Abuses”.
There is variation of course, especially in a socially and intellectually advanced species such as humans.
r-species strategies are still tended towards more in males, more in less developed countries and more in younger humans. k-species strategies are more socially acceptable and more clearly displayed, though probably still more common than r-species overall.
You will notice that the role of sex here isn’t to have many offspring, but for any given pregnancy to manipulate the maximum nurturing investment to make up for the greater physiological cost of reproduction to the female.
This fits in perfectly with what I’m saying.
The males here are being tricked and prevented from acting out more of an r-species strategy if they wanted to. They aren’t preferring to invest in the woman’s offspring in the case that its another man’s offspring. They might even want to pair-bond exclusively, but are likely being being prevented from that too - over their own offspring at least. There is also evidence of competition between opposite sexes at work here - either benefitting from tricking the other in the best way that they can get away with. This is in addition to competition between members of the same sex to mate with particular members of the opposite sex. The example you gave is in favour of the most physiologically costly female role in reproduction as I explained.
Viable but not necessarily optimal. Pair-bonding benefits the female more than the male - restricting the male to concentrating on just one female’s offspring instead of any others that he could do at negligible physiological cost from the reproductive act, which makes state monopoly over violence an inherently matriarchal institution, much to the benefit of less physically dominant males as well. But the opportunity cost of this strategy is lower than the more “barbaric” tournament species model at the current point in human history. The comparative advantage of higher numbers of less competitive male genes is higher for us than for lower numbers of only the more competitive male genes. For cultures that allow harems, this isn’t so much the case, and the fact that this more tournament strategy of harems still featuring in human behaviour nicely exemplifies how humans don’t cleanly fall into the k-species classification. These practices are more patriarchal, as more dominant males get to spread their genetics further and more reliably, and females get less individual attention from their offspring’s father than they would if the father was only investing in their offspring exclusively.
I don’t mean to oversimplify all variations to a crude homogenous allegiance to only the explicit core of all these variations, there are complications and overlapping distributions indeed - but they all play around the difference in physiological cost to males and females in reproduction, and they do form trends. They have to, or they will fall by the wayside in the face of natural selection. Natural selection only cares about what works, and is verifiably constrained by physiology in doing so. That’s not a Just-So story, it’s a risk/cost/benefit calculation.