In the absence of clear demarcations, it is therefore totally acceptable to simply make up any rule and either accept or reject people, based on these rules. Without telling anyone how you made those rules. And the rules can be adjusted to whatever you feel like, whenever you feel like it, depending on each person.
Part of the problem here is that you're confusing scientific findings with your preferred scheme of ethics. Whether or not you find the state of the world "acceptable" does not in any way impact how the world actually
is -- you may find it unacceptable that women cannot vote in some countries, but to deny that such countries exist is futile. You can even, if you like, work for political change in those countries in order to change things around.
You may also find it unacceptable that women are typically smaller and less strong than men. Unfortunately, that's another scientific finding that will not go away simply because you wish it. And in this case, I doubt there's even much that you can do to enforce change -- the differences exist because that's how evolution shaped human biology.
Evolution has also shaped human psychology and cognition. And one way that is has shaped people's minds is that people
do in fact make classification decisions based on sets of essentially arbitrary and stereotypical criteria (although, again, you don't seem to understand that these criteria are the exact opposite of rules) that may vary on an individual basis and that may not be articulatable. That's another scientific finding. Whether or not it's morally acceptable to
act in a discriminatory manner is an entirely different question.
We have, for example, strong biological evidence that humans have evolved to be moderately polygamous (and specifically polygynous). For most societies in history, polygamy was accepted. Today, most western societies consider polygamy to be unacceptable and demand that humans act otherwise, despite the evolutionary background. Similarly, many societies demand (with somewhat less success) that people act similarly to people of different "classifications," overriding the underlying biology. One of the nice things about humans is that they have that capacity to override their biology (to some extent).
But that doesn't mean that the biology isn't there.
