I wrote "it is not a girl, but a boy", to avoid the illogical sentence "she is not a girl", and I read that it's accepted and my sentence is refused? Oh my God, people can be weird!! To respect the fact that there is only ONE word (Ő) in Hungarian, my translation chooses as well only ONE word in English! Disambiguation is avoided as well my way!
English has an animate/inanimate distinction. "It" is inanimate and can't refer to people. "What" can't describe people when it functions as a pronoun. ("I'd like to meet what you're marrying").
(Adjectival "what" is okay, though. "Show me what men you are." This use is less common now.)
Using it/what to refer to people is generally very rude, not just a grammar error.
Hm... well, I think "it" should be accepted here. Sometimes when a couple is expecting a child and they do not know the child's sex yet, the couple refers to the expected child as "it".
"What are you expecting? Is it a boy or a girl?"
"We were expecting a girl. As it turned out, it is not a girl, but a boy"
This is just like the various Chinese languages. People understand through context, and there are other ways to indicate which person one is speaking about (e.g., that man, this woman, that child, these people). I haven't actually taken a count, but I think it's mostly the Indo-European languages that have a gender distinction for third person.