Silver said:
No, it's based on misogyny. As is the reasons behind why there are only male space marines. It's supported and defended by the lore, but that lore only exists because Games Workshop thought female space marines weren't as badass.
I'm afraid it's not. The XY gender has a greater fight/flight reaction, musculature around vital areas, less internal vital organs, shorter life span, better spatial awareness, greater average size, no downtime in race production; so is built for frontline troops. It's hampered by it's need to protect the XX gender, so it's mind is more focussed when not exposed to the XX pheromone.
The XX gene though is more resilient, lighter, greater at multi-tasking, faster ability to learn, capable of short bursts of immense destructive force and greater stamina, but equally hampered by the XY pheromone.
The thing needed though, would be research into the YY gender, to see if it is capable of surviving.
You can argue about the political and sociological issues as much as you wish, but biologically that's why any animal works like that. Frontline have to be XX, Specialists are often XY. If there is a glut in one gender, then they can be used as cannon fodder, but we're looking at the most biologically capable.
Dwarves or Elves differ of course, because genetically they're different, but I doubt a Harlequin still has their 'nads intact.