biology isn't negotiable
Indeed.
What makes us male or female biologically?
Let's just take the following (there are more)
External sex organs
Internal sex organs
Chromosomes
Hormones
So when we decide when a baby is born whether they are make or female, we solely base this on external sex organs. That's it. There is no hormone test, no checking of chromosomes or whether or not someone has ovaries. And usually that's ok.
But there are people that present as male that have xy chromosomes and people that present as female with xx. Or xxy or xxxxxy or other combinations of these hormones.
There are people born with a penis that also have ovaries, or people born with no penis that have testicular tissue surrounding their ovaries.
There are people born female with high levels of testosterone, and people born male that might have high testosterone, but have no way for their bodies to make use of it and so don't develop in the typical way.
These types of issues occur in roughly the same numbers as people that have red hair. Not common, but not rare either.
People that have this type of nonstandard biology have often been assigned the sex that they most appear like. In some cases its not at all obvious that there is an issue. If as they grow they identify as being the other sex, they are classed as trans.
I'm not saying that this is the case for all trans people, but it's clearly not as simple as 'biology isn't negotiable'.
In addition there are many cultures that either currently or have historically acknowledged 3 or more genders. The ones that used to and no longer do often became part of the British empire and were adopted into the binary system we perceive gender bringing to.
I'm not getting involved in debate here, I am not an expert. I tend to limit getting involved in these discussions because they can be so divisive and I understand why. But the headline issues (such as male/trans female in prison, trans female wins sporting event) skew the public perception of trans people who have no such agenda and just want to get on with their lives.