Women are a different shape. It's not just a matter of breasts, that's just an easy way to differentiate and you can't blame them for taking it. I get really, really ****ing sick of the 'they could be any gender under their argument' because in most cases they are in fact quite obviously male and it's an excuse not to make more effort*. FW even pulled this for ****ing Imperial Guard for gods sake. Look me in the eye and tell me a Cadian could be mistaken as a female. Women have smaller waists, wider hips narrower shoulders, smaller hands and feet, longer upper arms and quite a few other small things.
The fact is none of the Aspects have any female soldiers in them, you can assume all you want but they are quite clearly and distinctly males. You can get away with it to some extent if you add a female head to the body but that's still quite unsatisfactory. If you read the Path series you will read that the armour of each Aspect Warrior shapes itself to their bodyshape, which means the differences in female form should be obvious. They aren't on the models because the models depict males. That's all their is to it I'm afraid.
There were rumours the SS Phoenix Lord was going to be female but sadly not. I've actually written to FW to suggest they retcon it in the pdf update but they won't of course. Apparently for a race that has no distinctions between gender and soldiers are just as likely to be male as female there is still only one female Phoenix Lord out of nine.
*Crankiness aimed at GW and FW, not either of you.