Dororo has been raised a boy by her parent(s) and that's the only reason she identifies as a 'he'. Purely because she doesn't know better, hasn't been treated as a girl by anybody in her life and most certainly not because SHE wanted to BE A BOY.
Perhaps her parents raised her like that to keep her safe from predators but I doubt boys that age would be that much safer in those times.
How difficult is it for some people to fathom that characters such as these do not follow the same twisted 2k19 ideology as they do?