*lets out a long sigh and drags out her soapbox*
Out of everything in the 30/40k universe that people accept without question- all of the deamonic entities, xenos species, psykers with god-like powers, lobotomized human-machine hybrids, perpetuals, Primarchs, etc...
The fact that seems to come up for argument over and over again is that females CAN NOT be astartes.
Let me be clear here, the lore does not leave wiggle room: Astartes are, and can only be, male.
'But, but.. what if *insert whatever lore butchering left-field concept that the arguer has convinced themselves of*...'
No. Just, NO. The lore states directly that the process fails if tried on a female. Not sometimes work in rare cases. Not one in a billion chance. Fails - and fails catastrophically at that. Why does it fail? It fails because the process doesn't work on those who are genetically female.
Why is this so hard of a pill to swallow over all of the strange and bizarre facets of 30/40k? You want boobs on your army, got it... Sisters of Battle are that way
. Yes, I know they need an update. I hope they get a glorious one sometime soon, with enough plastic models and bitz to choke a bloodthirster. The fact that their models are less than stellar right now is no reason to take a chainglave to nearly 30 years of established lore.
*kicks soapbox over*
Now, onto the second topic that has reared its head in this thread. Astartes and procreation.
The Crimson Fist book - Rynn's World touches on this topic. Apparently, one of the older chapter masters (Traegus) had attempted a breeding program with failed
aspirants. It is noted in this program that only aspirants that failed early in the process to become astartes were viable for the breeding because after a certain point in the process (if they even survived failure) they were rendered sterile. This breeding program failed so spectacularly that is was not only abandoned but was banned from ever being attempted again.
Now, there are some Chapters that encourage their warriors to live
among the people of their home world (Salamanders are a great example of this). These Chapters tend to emphasis protection of humanity and 'living' amongst the human population helps to remind the trans-human astartes of this mentality as it is apparently easy for astartes to 'forget' their humanity and view normal mortals with disdain.
From everything the lore hints at- astartes cannot breed (be that from lack of viability to lack of desire). But, the lore has notable instances of astartes becoming 'attached' to females in a protective sense. This can be seen in a number of storylines- from Ragnar (40k Space Wolves) to Argel Tal (30k Word Bearer). It seems that physical attraction and desire is mostly wiped out from an astartes through the indoctrination process and any lingering traces that remain convert into a fierce desire to 'protect' the female in question- much like a highly trained attack dog 'bonding' to a certain individual.