Fascinating perspective, well expressed.
One thing I would clarify is that there are still many different cultures in existence. Although most cultures are converging due to the global economic hegemony enforced by the US, they still maintain highly significant differences.
For instance, in many Muslim countries, your argument wouldn't apply as much for a wide variety of reasons, including the prevalence of arranged marriages.
Furthermore, each generation actively produces its own culture and it can sometimes change rapidly due to changing environments. I agree with you that culture is built around human biology and in some ways remains similar across all human communities regardless of time or location. However, within that general framework, the possibilities are almost infinite, as we can see just by observing history.
So, in this specific context, I would argue that while it's essentially inevitable that men will take on the more dangerous and difficult roles in any given culture, the actual manifestation of that tendency can come in many different forms. Western society manifests the male disposability phenomenon in a particularly harsh manner, in my personal opinion.
I think that many other cultural lineages may have traditionally held less demanding/dangerous expectations of masculinity. A relevant factor is that all Western nations have military traditions going back millennia, whereas many other regions of the planet do not share such an extensive history of warfare. All Western cultures essentially trace their roots back to the Roman Empire, in which basic mechanics of the male gauntlet which you speak of had already been firmly established.
And your internal gender is something that you automatically just know? Have you always known? How?
It seems like at some point, some external reasons may have been involved.