Believe it or not, this IS a serious question...

[GiqueCee]

41. When I have to respond to a MAGA moron on this subject...

... I ask them, "Do you know why men have nipples?" It is not a non sequitur. I then point out exactly what the OP said, that every human – in deed, I think it's every mammal – is FEMALE at conception, and that the Y chromosome does not manifest itself until later in the gestation period.
Like every other phase of fetal development, the timing of these changes is infinitely variable. It is why people are born gay every day, and some exhibit gender dysphoria quite early in childhood. Simplistic assumptions, like those of the Malevolent Man-Baby currently defiling the Oval Office, are the product of weak minds too rigid to accept or assimilate new information, as is the case for every conservative that Satan ever defecated.