Okay first off notice the use of the word 'theory' this is not truth and you can believe what you want of it this is just my opinion.

KuroYuri is first off in the army we have only seen one femael in the army so far out of tons we have seen, this makes it likely he is a boy.

KuroYuri got into the church to take the bishop's exams as far as i know bishops can only be boys. This further supports why you see no female test takers and also why you see no bishops that are female, although in the animê i did see a few ponytails but they could have been guys since i couldnt see there face's.

KuroYuri displays mais of a boyish personality then a girlish one, sure he has rosa, -de-rosa hair but really, in the animê world you can have whatever kind of hair you want lol.