Once you redefine gender as different from biological sex. It can refer to whatever, the current ideology defines gender as meaning. At that point is serves largely as a social convention used for whatever purpose such terminology is intended to serve. Historically, it was used to assign societal roles.
However, the different sexes have one singular purpose to reproduce the species in a manner that uses natural selection to strengthen the society. So, it produces both a more diverse genetic population along with more numbers. However, because humans take quite a while before they can operate independently, a family structure emerged to insure children receive the necessary care to reach maturity. This family structure was comprised of both sexes to produce children, but not always in equal numbers.
Because one male can impregnate many females, but only females can produce offspring, historically females have been regarded as a "societal asset" to be protected, while males while still valuable are expendable. This "protection" came with benefits and disadvantages.
In other words, the "natural" relationship between men and women was largely grounded on what served the needs of society best. Not what served each individual or even each "sex" best.
Simple example, take a small tribe of 100 adults, 50 males and 50 females. If 40 males die in war, the 10 males remain and along with the 50 females, the group can replace the societal loss in one generation and even add additional members.
In contrast, if the same society lost 40 females, it might take multiple generations to replace the population especially in the past, when so many women died giving birth to children.
In modern society, the need to produce children has diminished greatly. And so, women are transitioning from being "protected" members of society to being thrust into wars, the workplace, and other higher risk activities. Arguably, some women prefer this role to raising children and that works in modern society (though frankly at today's fertility rates), that might change over the next two to three generations.
In other words, biological sex is largely linked to the reproductive process that divides people into one of two sexes. But gender largely existed to provide societal parameters in which individuals could operate.
When reproduction was one of the most important activities to preserving society, sex and gender came to be largely equivalent. But gender at some level was used to define what people's role in society was.
Those roles have become blurred as reproduction is no longer is as critical as it once was to society. That said, the redefining of gender based on self-identification, does not serve our society well and so eventually a better definition will emerge, that is more useful.
Ultimately, gender to have value as a societal definition must serve a purpose. For now, it no longer does. But neither do the old definitions work.
