Okay first off notice the use of the word 'theory' this is not truth and toi can believe what toi 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 toi see no female test takers and also why toi see no bishops that are female, although in the animé i did see a few ponytails but they could have been guys since i couldnt see there face's.

KuroYuri displays plus of a boyish personality then a girlish one, sure he has rose hair but really, in the animé world toi can have whatever kind of hair toi want lol.