Long story short, the founding moment of the legion relies on a total impossibility. Geneseed is DNA and tissues that, when correctly cultivated in the most strict laboratory conditions, can be grown into a dozen organs, each with their own DNA structure seperate to that of a human. Then, to even be in a person, they must be subject to a vast aray of surgeries and augmentations, and even then it's still almost entirely luck-based. What ou've got here is a pile of oreganic mush that, with nothing to support its on life whatsoever and exposed to the elements, manages to utterly inexplicably survive, then grow an entire organism whose DNA it does not contain around it and implant itself into all the suitable locations for that organism to be a Space Marine, all without any means of generating the energy to do this or protect itself from lethal exposure. Add to that that the fact that Geneseed doesn't naturally do anything at all, and you've got something less likely than you winning the lottery every day for the rest of your life.
Also, Space Marines lack the will and apparatus to breed conventionally.
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