The issue with female in the frontlines is not that much an issue of physical strenght, albeit it's present. Ultimately a fit female is just as good as a 60 years old man or a young teenage boy.
The real issue is that humans are hardwired to protect and help females above anything else, including the mission. In WWII, mixed USSR regiments registered far higher casuality rates than non-mixed regiments, as well as sanitary issues (STD's and pregancies).
So while having female support troops (Artillery, clerks, medics, hell, flying personel, etc...) make perfect rational sense, the only armies in the world that have female grunts are either armies that don't expect violence to be used against them (Lol Sweden) or Insurgency groups scrapping the barrel.
Also, from a troop moral perspective, knowing that female POW of your units are gonna be raped if captured is not a good thing, and an absolute no-go for modern western militaries.
In the context of 40k, when the opponents are xernos or are gonna rape you anyway (if not both) and where the death ratio doesn't matter, mixed units kinda makes grimderp sense.