Let's dissect this argument on an army-by-army basis, shall we?>Space Marines (including all derivatives )
All male because the Emperor willed it so, pretty much the only all male force canonically in-game.>IG
Imperial Guard recruits both genders, depending on what world you're talking about.>Inquisition
Recruits both men and women>Sisters of Battle
All-Female, all the time, like a distaff counterpart to Space Marines>Chaos Space Marines
See: Space Marines>Orks
Neither male nor female, breed via spores.>Tau
Male and Female Fire Caste Tau fight.>Necrons
Male and Female Necrontyr were equally biotransferred.>Tyranids
No genders as far as I can tell.>Daemons
Ideas and emotions made manifest are hard to qualify as having a sex.>Eldar and Dark Eldar
Both are equitable societies when it comes to gender.
Are there any more I missed?