N0-1: At no point have I said that every last one of them does. I only quoted existing fluff that every 21st founding chapter that currently survives has some form of mutation. And that we're missing... 25 chapters (approx) of that founding that we have no names for whatsoever or any indication of what mutations they have. (Havn't bought the Badab book yet, my book budget was tight and I had a choice between DW and Into the Storm or Badab.)
Kallador: they dealt with it by the women playing guys. It's unknown how much of the decision was GW insistence and how much was author/editorial preference.
HBMC: It's nice to know I get up your nose. However, you do pretty much the same thing on every thread on this subject, no matter who posts to it, so it can't just be me. And as far as 'All Space Marines Are Male' being an absolute of fluff, I have another fluff phrase for you:
"Everything you've been told is a lie."
You know, one of the central underpinnings of 40k fluff? The fact that the supposedly canon fluff itself is not reliable? (or the built in Author Saving throw that's used to justify the occasional radical retcon.) The same thing that's used to brush off fluff that's not fluffy anymore? Half eldar astropaths, Space Marine Inquisitors, IG Jetbikes... squats, anyone?
This IS a setting where fluff is supposedly composed of half truths and outright deceptions, after all.
And, most things say nothing at all on the subject. The only places the male 'requirement' is mentioned is: The Index Astartes artical, and the DW Corebook. (that I've been able to find, granted, since it's not in the current codex)
As far as FSM in fluff: Other then the woman turned into one (somewhat) by a daemon in Storm of Iron, (CSMs did not notice the difference between her and the man she replaced other then to state that she seemed deadlier for some reason.) I can't think of anyone else that arguably turned into an FSM.
I've asked people, including a GW employee, 'if a peice of fluff, such as creation of a space marine, does not appear in a new codex, where a new version of that fluff is written, does this trump previous WD/Codecies/GW licensees?" With out exception the universal answer was 'yes'. As soon as I said 'female space marines' the GW employee clammed up, and the 40k players I talked to started screaming (granted YES! and NOOOOoooo! were about equal).
Edit: DAMMIT: Exark beat me to the 'Everything ...is a lie' quote.