Leaving aside the specific problems of a male murder being misclassed as a women (stats, prison, disyortion of public understanding of who is a risk to who etc)already flagged by many PP...
It is impossible to consider a person with a male body to be a "woman" without subscribing to the belief that men and women have different types of mind.
In other words, to believe a man is "really a woman" is to believe all women have a certain type of mind which is different to all men.
That is a sexist, reductive belief that harms and diminishes all women.
Firstly, because it reduces our personalities and mental potential to a sexist stereotype.
Secondly, because it obscures the real truth that we are treated differently by society not because people somehow see our "womanhood of mind" but because people see and react to our bodies.
Thirdly, because it appropriates the mitigations and protections originally put in place because of how society treats people with our bodies and redeploys them as resources for people who identify as having womanly minds, thus forcing us to either accept the imposition of also being defined as someone with a "womanly mind" or lose access to these resources.
Fourthly, it takes away the formerly female only words we need to protect ourselves in law, make our case politically, or indeed talk about the issue at all.