Ok, this twins thing is a great example. In this case they are both mixed. Simple.
One will likely be called black from time to time and the other will likely be called white from time to time, because these words are often used as decriptors of a person's appearance, but as we've already agreed, getting to the absolute truth in this case would require DNA testing (if anyone cared enough to check).
It would be inaccurate to describe either of them as Nigerian or as Norwegian, they're both a mixture of the two and it just so happens that those mixes have come out looking significantly different (which isn't typical at all by the way, let's not pretend it is).
But, and this is important, it would also be inaccurate to describe them as Asian, because they're not. Even if they decided that they felt like they were. The fact that there might be ambiguity about their lineage at the surface level, doesn't mean that anything they like can be true.
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I recognise that gender doesn't have to conform to biology because gender is behaviour. Gender is to sex as culture (or really cultural stereotype) is to race.
The fact that you're biologically male doesn't, or at least absolutely shouldn't, mean that you can't wear a dress or get your nails done or engage in any other stereotypically female gendered behaviour. The fact that you're white doesn't, or at least absolutely shouldn't, mean that you can't put your hair in corn rows, or listen to rap music, or engage in any other stereotypically "black" behaviour.
But the behaviour you engage in doesn't change your biology. If you're biologically male you still have to worry about prostate cancer more than a biological woman and about breast cancer less than a biological woman. You'll still likely be bigger and stronger than the average biological female of similar ancestry.
These aren't mere cultural norms or "man made constructs", they're biological facts. We don't have to pretend they aren't in order to vigorously defend the rights of all people to live their lives in the way they feel comfortable.
