It does not matter if biology claims trans women are males.
Biology is not concerned with the violence done to people. Biology is not a shield to do violence to people, and indeed, the admittedly flawed models of colloquial biology often cited against trans women have also been used to justify and make excuses for violence against minority populations in oppressive systems.
Violence is still violence.
It is still immoral, still unethical, and defending it is immoral and unethical.
Psychology, sociology, anthropology, physiology, medicine — these sciences have all proven that calling a trans woman a man is violence.
So it doesn’t matter if biology says male when biology, itself, is being violent, according to other sciences.
Because that violence is still violence.
Violence is not limited to broken bones and bruised flesh and physical damage visible to the seeing (an ableist concept itself, so compounding the violence there).
It is words. Ask those fleeing persecution, read history, talk to survivors of child abuse and domestic violence and prison violence. Words are just as physically damaging - and according to many measures more so, since the brain treats those words no differently than it treats the body blows. It sends the chemicals out to the flesh and the organs and it sears synapses and it lasts long after the bruises and the broken bones have mended.
It has physical, measurable consequence, and it endures and we know that this applies even when it is strangers. The science establishes it.
This is fact. Not opinion.
Calling a trans woman a man is an act of violence, an assault, and those who do so are being violent, are being immoral, are being unethical. Silence in the face of violence is complicity, especially when that violence is social. Defense of calling a trans woman a man is defending violence.
Liking it, re blogging it without calling it out, these are forms of complicity. If you cannot tolerate violence against a woman, yet you can against a trans woman, who is also a woman, what sort of a person does this make you? What sorts of lessons are you teaching? How can you only oppose violence against some women?
Name it what it is. Don’t dress it up, don’t reduce it, it is violence. It is unethical. It is immoral. Shame those who do it, teach them it is wrong.
Because not doing so means you are complicit, means you are supporting, means you are not trying to stop violence against gay, lesbian, bisexual, and even straight people.
It means you are not trying to stop violence against people of color, against immigrants, against the disabled, against the poor.
It means you are standing by watching as someone does violence to another person. And that is immoral, unethical, and shameful.
Regarding why trans people say they have always felt like x or y, what they are trying to express in colloquial and layman’s terms is the concept of gender identity, now more commonly referred to as either social sex identity and physical sex identity, which it two concepts, each distinct.
Roughly translated, the whole thing means that a Trans person is aware that they are a woman, man, both, or neither, at the same core level as they are aware of themselves as a person, distinct from other people.
Some people would prefer to argue that what this is suggesting is that there is a “brain sex” — and yet, that’s not what this is describing. It is describing a sense of self-awareness — which, while decidedly part of the brain’s physiology, is really part of the existential notion that one exists, and therefore is not part of that argument, nor even related directly to biological systems (though many studies indicate such, I’m not going to reference such as biological essentialism of that sort is somewhat contrary to existential systems such as Transcentrism).
What that means, as well, is that this description has nothing whatsoever to do with biology, and, therefore arguments about biology aren’t valid when contradicting it. It would be akin to saying that the United States army shouldn’t exist because of broccoli.
This all comes together in the core aspect here: like sexual orientation, this is part of what creates the sense of self in people, and it therefore is part of how they know that they exist as an individual human being, and is a part of that.
So the same question of how you know can be answered in as many different ways as there are people, and it is the still same way that those asking the question know what their social sex identity is.
This effectively makes it the trans version of the question “how do you know you are gay?”, with the same accurate answer “I just do, because I know I exist.”
The answer may not satisfy those seeking some sort of concrete answer, but it is inevitable that they themselves, will discount any answer because for them the same question remains unanswered in a way they would accept from a trans person, usually due to cis privilege.