I suppose the 'proper' reason for transgender ism is a medical one - a strong feeling if having been born in the wrong body. That is the medical reason for transgenderism.
No, that used to be the medical definition of transsexuality.
Transgenderism is all about what genderfeelz you have. Biological bodies don't come into it.
That's why we now have headlines like "man gives birth" ... someone feeling strongly that they should not have been born in a body that is capable of giving birth would not have a planned pregnancy and then talk ot the press about it, even.
Now, most transmen (i.e. females with male genderfeelz) do bind their breasts and take testosterone, which does not surprise me in the least because hating your own body has a long tradition for women, and women have always bound and restricted their bodies; see corsets. (Nuns also bound their breasts in the past, not sure it is still done)
But it is quite obvious in most transmen that hate of their own body was not the beginning, it was an effect of internalized misogyny. Read the LGBT teen forums here on mumsnet; women report their daughters suddenly identify as trans - girls who never mentioned any discomfort about their genitals before puberty.
Also, all the middle-aged lesbians (it seems to mostly be lesbians at that age) who suddenly trans themselves, with surgery and testosterone - were they dysphoric about their bodies? I don't know whether they had happy sex lives, but considering that many were out and proud lesbians, I would assume they did engage in sexual activity with their female genitals.
Also, ALL transmen who belong to the transwacktivist lobby group (which I would wager is almost all of them) agree with the notion that penis can be female, and support the male trans who complain about the "cotton ceiling", i.e. not getting their dicks into lesbian panties. As male trans are much less likely to have surgery than female trans.
All of those transmen also think that a woman becomes a man by declaring herself so. They see their body modifications as individual choice, not as something that must be done. Dysphoria about one's body is not required to be trans.
I think that is because subconsciously they know their body dysphoria stems from internalized misogyny, and they were not born with it.
They can see that those women in history are just like them - but as they cannot acknowledge themselves to be women, they instead redefine those women in history to transmen.