Y'know how Space Marine Initiates have to be around 6-12 years old to be taken in, or otherwise the process will not work properly? Well, in the 30th Millennium, there were a few cases of people being inducted into the SM Legions without being of the proper age. These people are described as being "False Astartes", one of the more notable examples being Kor'Phaeron, who is Lorgar's adoptive father. Instead of the usual steps taken, said person will undergo genetic modifications required to be considered a Space Marine all but traditionally by a combination of bionics, rejuvenating surgeries and 'limited gene forging' (incidentally, this is also something you can do in RPG Games such as Dark Heresy and Rogue Trader). Now, imagine this - what if Kor'Phaeron was an adoptive mother instead of a father? Bionics, rejuvenating surgering and gene forging have nothing to do with gender, and there is no geneseed involved. And Kor'Phaeron was only allowed to enter into the Word Bearers because he found Lorgar in a crater and adopted him so he got super special privileges, not because he was an especially fierce warrior so they just HAD to induct him into the Legion. It is a total possibility without a single broken lore. Still, said person would not be a real Space Marine, and 'False Astartes' are said to be mistrusted and resented - but this still is one possible way to actually include a female into an Astartes Legion. Although, I imagine there'd still be nerdrage over it because girls are yucky and all that.