Now, whether or not female space marines make thematic sense in the world of 40k is a different question that I'm not sure I'm equipped to answer, but from a pure world building perspective there's literally nothing stopping female space marines beyond GW's disinclination.
Put crudely, there is no thematic to space marines in particular or 40k in general that is so intricate it will not survive the inclusion of a double-X gene.
Alright. For real. Anyone reading this, come a little closer.
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No, closer than that.
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No no my friend, I mean closer.
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Closer.
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OK a little too close.
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That's good.
I'm going to share with you the secrets of GW's space marine creation process. Every step and secret to creating space marine characters and their chapters. And for free, I'll throw in the Kuja-patented extra secret step (works every time) to making female space marines.
How to Create a Space Marine:
Step 1: Pick from one of the following: [influential figure from general history][figure from military history][figure from ancient myth][figure from classic literature][figure from scifi/genre fiction].
Step 2: Change the letters if you're feeling saucy to disguise the reference or make it cutesy so that nerds see it and go 'aha, I recognize that reference!'.
Step 3: Put them in a chapter that fits their thematic reference.
How to Create a Space Marine Chapter:
Step 1: Pick from one of the following: [really cool historical warrior sect][really cool mythological warrior sect][really cool scifi/genre fiction warrior sect].
Step 2: Change the letters if you're feeling saucy to disguise the reference or make it cutesy so that nerds see it and go 'aha, I recognize that reference!'.
Step 3: Give them a homeworld and equipment and fighting style that fits their thematic reference.
You have now Done A Space Marine the Games Workshop Way. Congratulations! Now, as promised, here's the secret special technique for adding female space marines into the mix:
Step 4: Refer to as many as you want with she/her pronouns.
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It is literally that easy. Here's an example you can follow at home:
View: https://flagcaptainart.tumblr.com/post/177784415205/a-commission-for-starcunning-of-their-and
Antigone (left) belongs to my wife. Euryale (right) is mine.
Our Wrath and Glory campaign was based around the Ultramarines creating a new successor chapter, with veterans being drawn from several existing chapters to helm the new organization. Antigone came from the Ultramarines themselves, a chaplain sent to ensure discipline was instilled in the new recruits and that the Codex Astartes would be followed properly by the emerging chapter. A Calth-born recruit herself, she is strict and orthodox. Euryale came from the Iron Snakes, and was recruited as one of their respected apothecaries. She is known as 'the Stone-Faced' due to her disinclination to show emotion.
Antigone is of course taken from the famous play, while Euryale was one of the three gorgon sisters (picked because Medusa is over-referenced).
And that's it. That's literally it. Female names and references instead of male ones. This is the supposed destruction of 40k's thematics. Give me a fucking break.
I also want male sisters of battle, for the record.
For all intents and purpose, they already exist, they're just not represented as a unified army on the tabletop. The Sisters were preceded by the Frateris Templars, who got BTFO by the space marines during the Age of Apostasy and so aren't fielded in mass numbers anymore. That doesn't stop figures like Witchfinder Tantalid from the Eisenhorn series from existing, or the fanatical
Crusaders you can use as part of an Inquisition army as the church tries to skirt around the Decree Passive (mentioned on the lexicanum article).
(I've always found it funny that the Sisters are constantly mentioned as the equivalent of the space marines when in fact they're not a wholly-female army - they've always had male priests as HQ options and for the longest time one of their two heroes was Uriah Jacobus, although I think he's finally been
just recently retired from the TT. 9th Edition has gone a long way to actually make the Sisters all female [not least of all finally adding more HQ characters alongside Celestine], although the priests are still in there.)