During that time period it was much safer to be seen as a boy/man than as a female. No idea if that's why Dororo does it (probably not, because even as a child she did the same.) But to say it's an abomination for a girl to dress like a boy or the other way around is wrong. Sometimes the situation requires it, and even if not, it's the person's choice. If you don't like, that's on you.