The question Is "Allah" a he or she? points out that referring to Allah as "he" is a language matter
...unlike English, the Arabic male pronoun does not imply gender. English, quite frankly, does not have an appropriate pronoun that fully encompasses this. -- goldPseudo ♦
(See also Is Allah a male or a female?.) This made me think that it wasn't such a big deal referring to Allah as "He", since it's just an arbitrary convention.
However, one can find fatawa which are against referring to Allah using the female pronoun "She". In an extremely strong tone, Islam Q&A write:
Hence you will find that most of those who believe that there is a God above the heavens refer to Him in the masculine, which is more befitting to Him, may He be glorified. This is something natural that does not need any research, examination or evidence. ... If anyone were to refer to Him in the feminine, as this poor girl said, he would be opposed and accused of ignorance and clear misguidance.
At the risk of requesting research, examination, or evidence...
Question: If Allah is neither "He" nor "She", why is it so important to call Him "He"?
The Islam Q&A fatwa's tone is so strong, they basically describe this as kufr. However, aside from a "proof by intimidation", they don't seem to provide substance to their claim (particularly for a claim phrased so strongly).
It's possible the answer basically boils down to this part of the Islam Q&A fatwa:
Imam at-Tabari (may Allah have mercy on him) said: ... the feminine of any thing is inferior ...
Shaykh Ibn Sa‘di (may Allah have mercy on him) said: ... If the names are feminine and inferior, that may point to some inferiority in the thing that is thus named ...
If it's the case that we can't refer to Allah as "She" because femininity is perceived as inferior, this question becomes one of confirmation.