A note on the elite of elite of elite Imperial orders being exclusively female or male:
40k plays on archaic strings. It's better worldbuilding by being more archaic by having the exclusive elite warrior orders mimic monks and nuns and be separate. Female Sisters of Silence and Sororitas on the one hand, and male Astartes and Custodes are much better worldbuilding ploys than mixed orders of Astartes and Custodes. You don't mix monks and nuns and retain an archaic impression.
This is one example where GW has stayed a lot truer to the spirit of 40k through all these decades, than one would expect. Kudos to GW for playing the right strings to build their setting, where so many others would have fallen for outside pressure and muddled the setting.
Of course, the elite monks and nuns situation does not apply to the Mechanicus/Titanicus (who cares little about fleshly matters) or the ragtag plebeian hordes of Imperial Guard (where any setup conceivable, such as mixed or separate regiments, or just male or even just female regiments will happen somewhere depending on local culture). Neither does the Inquisition need it, since it's such an excentric individually focused organization. Sororitas/Astartes and Custodes/Sisters of Silence is the relevant arena. They are the big shining warrior orders.
And they ought to feel archaic. This isn't the Dark Age of Technology, but the regressed, myopic and parochial Age of Imperium, where things often do not make sense and weird traditions are king. There is a good reason why Games Workshop in the 1990s abandoned the idea of female Space Marines and gave the Sisters of Battle a true remake into their very own cool thing.
Cheers
