This is just it.
We see the exact same thing in other places and other contexts where there are benefits conferred to individuals for memberships in groups deemed to be opressed - not class (ecoomic) groups, but racial, caste, ethnic, sexuality related, or other groups.
This what has diven a lot of recent controversy in North American First Nations communities about who counts as indiginous. It'a actually really hard to define - is it like citizenship (this is how the Cherokee treat it.) It is DNA, and if so, how much. Is it about living or growing up on the rez? Something else?
These questions used to be somewhat academic, fakers weren't looked upon positively, but they didn't get much in the way of concrete benefits for it. If anything people felt the benefits of indigenous heritage were subject to too much gatekeeping, so for example children adopted out wouldn't have access. So there was an effort in various contexts to expand.
But now what we see are all kind of people trying to dig up tenuous connections, or even fake ones, for access to things like jobs, internships, and so on. And the more lax the criteria put in place the more people do take advantage, it's a huge issue in academia.
All of which has ignited a huge debate within and between indigenous communities about how they decide who is indigenous.
This is the reason that if you have a place like India, where they want to try and create concrete benefits to help low caste individuals, they need to have considerable safeguarding in place to make sure others are not fraudulently claiming them. And trying to nail down caste is not simple, much like trying to decide who counts as indigenous.
The comparison to sex is interesting in that we have people trying to claim that it is too complicated, and not worth trying, when we see other examples of attempts to do this kind of thing that are far more complex. Not just in India either, left wing progressives in the west are all for this kind of thing in every area except sex. Because they want to push things like race based benefits and affirmative action, and that requires defining the categories. They know this.
I'd question whether some posters are really pushing back on this because they "don;t understand." I suspect that they want to continue to support these kinds of programs but don't want them compared to things like government benefits based on caste, because it's a little too close to the bone in terms of how it rarefies these socially created categories.