Its implied that the shinjus fruit cursed her and she's a hannya demon. Search it up on Google...the hannya mask looks very similar to the death god mask that oro used to summon the death reaper.... Regarding her and the reapers features.... The reaper is skin and bones like a mummy and if you've seen a mummy there's no real difference in the appearance of females and males.. That's why they call in forensic scientists to determine the gender of a mummy...
I've seen no reference in the manga to a curse on Kaguya. But if I've missed something, by all means. Aside from that, while the Shinigami is obviously emaciated, it's hardly mummified. It's features are still identifiable. In fact, it's form looks exactly like Nagato while he was linked with the Gedo Mazo. There is absolutely no reason to think the Shinigami is not masculine.
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