Actually, in the manga, Crona's gender is female (as far as I know).
But Crona's gender is unknown, because Patty said she couldn't tell, and Black Star asked Crona if he/she has a ''Man's Sword''. And, 'Boku' is also being used by girls, mostly tomboys. Not as often as males, but it's still used. Plus Crona sounds (and somewhat looks) like a girl, but I still dunno.
I can't deal with girls, either. My gender frightens me :3
CRONA, YOU NEED TO LEARN HOW TO DEAL WITH YOUR GENDER.
I did see one fic that did make androgyny work, with Crona having been the result of one of Medusa's experiments (on herself to create a child as a personal killing machine). So I would say to try to make no real biological sex "official", and base the mental gender on however you interpenetrate the character. Just be mindful of others.