As for FSM... Remember that canonically Malcador the Sigilite joked with Emperor about making some female Primarches to "easen the tension" (albeit I can see it easily causing disturbance due to love and assorted chucklefuckery) but the Big-E got all serious and said it can't be done because the technology just isin't here yet. He's the God-Emperor, he conquers Galaxy and knows things, who are we to argue? Well, I mean, it's not like Female Space Marines woudln't be cool... after all,
love is a battlefield!As for Marine we-wees, I belive the general idea is that it depends on the Chapter in question, wether they actually bothered to cut off the initiate dick or leave it there... as for being sterile... majority of Marines begin the process before adolescence so it's not exactly out of question that the ridiculous hormone bullshit that people tend to not survive might screw up with their balls content. I myself belive that it might happen, but they usually don't have chance to do so (they go around being busy and killing heretics all the time), they could not really want to (religious indoctrination bullshit), not all of them can (dick cutting) and even then children are very unlikely... and it's not like the kid would be special too, mostly just a curiosity to Inquisition. It's just easier to let all the women (especially the sex-deprived Sororitas, though in that case the kid might actually be special due to Warp fuckery and intense faith) in Imperium belive that Marines can't fuck so they won't bother them.
As for what Gene-Seed already is... I belive I already ranted about it few pages ago. Grim has it right, and going by Tacks logic pubery is actually gene-fucking too, because body grows tits on it's own. As for the "vile corruption" - it's Warp, it might as well depend on indoctrination, and there's actually a huge chance it does, because some Guardsmen actually can resist it pretty well, and it happens that the Guardsmen in question are usually from the most brainwashed and awesome regiments.
As for power fisting Avatars of Khaine... consider that in some lore Lasguns are mentioned to be capable of penetrating 10+ meters of reinforced concrete or pretty thick metal plate
on regular setting. It all depends who you ask, and until we get a truly objective simulation of WH40k warfare, just go with what your headcanon accepts.