I've never liked the "born in the wrong body" framing of transness, because 1. it feels like it assumes a level of sexual dimorphism in brain structure far stronger than we have any evidence for, and 2. the word "wrong" has a kind-of moral component to it that I don't much like. But I think I've realised that there is a way that it actually makes total sense.
Even if brains aren't particularly structurally dimorphic, they are still implicitly gendered by society. Your mannerisms, interests, tone-of-voice, gait, your whole personality in fact is gendered whether you like it or not. And the sense in which it can be "wrong" is that society can see what it considers a mismatch between your sex-at-birth and how it genders your personality.
E.g. a "boy" can be gentle and softly-spoken, not necessarily because of "his" brain structure being "female-typical", but for any number of potential reasons, and whatever the reason, society will gender "his" personality as female (albeit it will not always use the word "female", but more likely a different f-word to describe it). And then this mismatch between this and "his" birth sex creates a moral problem: it's wrong because society says people with bodies like that should not have personalities like this. And society's response to this moral problem is transmisogyny.
Trans women are often accused by transmisogynists of having archaic views on gender, but the idea that gentleness, meekness etc are "female personality traits" is not a claim made by trans women, but merely a description of a belief genuinely held by almost all of society, and that belief is one of many sources of our oppression. You (society) are the ones telling me I have a female brain, and you are the ones telling me that's wrong.
I could have exactly the same brain, the same personality, and if I'd been born with a different birth sex my life would've been so much easier. In that sense I was born in the wrong body. The body that I was born into has made everyone treat me as "wrong" my whole life, my body is wrong because everyone acts like it is.
Society's preferred response is for me to change my brain, my personality, to become the man I was always supposed to be. Next best is to do nothing, just carry on being the softly-spoken "boy" who keeps "his" head down and does what "he" is told. What society really doesn't want is for me to do the opposite, to change my body, to lean even harder into the womanhood for which I have been denigrated for so long. To say "Yes, I am a woman, I am claiming for myself the gender you have been applying to me all along."
To do this is to take the body that was "wrong" and do something even "wronger" with it. To maintain what society wants me to change, and to change what society wants me to maintain. My womanhood is a response to their misogyny, to take the part of me they tried to crush and fully embody it.
