I say don't overthink it. If it's two physical girls, it's yuri. If you begin worrying too much about the mind inside, it gets weird quickly because male and female slowly become less distinguishable there and you wind ultimately up having to consider whether tomboys can be yuri. Besides, we're all both genders to some degree on the inside.
Mind you, I'm not contending that gender bender girls love is not-straight. I'm saying it's yuri and straight if it's a gender bender. Both at the same time. Arguably the best of both worlds, for those of us who love gender benders.
EDIT: I wouldn't tag this manga as yuri, though. Not as yet.