40k lore went through so many changes, the stuff about only men being able to be marines is not really the most essential part to the story, is it? I guess its not really something they can change on the whim, because if there are female space marines there should be female space marine characters too, probably.

Also every superfan of warhammer Ive heard told me that Games Workshop sucks so maybe homebrewing your own stuff is okay. Maybe you could come up with the lore that it works with a modified geneseed and your own special chapter is special in their own special way because of that