Quote Originally Posted by LordCdrMilitant View Post
As I understand it, it's definitely possible for the opposite to be true [that there are female space marines but all have XY]. But in order for the genophage that modifies the person into a space marine to work, they have to have a Y chromosome. Presumably, it uses the fact that the Y-chromosome is always expressed [whereas, in AFAB people, one X chromosome is actually dormant and one is active] and tiny to add all the other space marine gene stuff to the end of the Y chromosome or something and guarantee that it's expressed while having reduced potential to malfunction due to interactions with also-active matching genes on active chromosomes.

OTOH, both statments are probably be true in the Emperor's Children.
This is me trying to enjoy 40k while keeping the 'no female space marines' rule, I took the idea that they get heavily modified to say that all secondary sexual characteristics are essentially removed, and that the indoctronisation process is designed so that by the end of their training all space marines will identify as male, with those who don't being disposed of somewhere along their training. To me it's a suitably grimdark explanation that doesn't require me to wrap my head around the idea of MtF people not being female (which I have massive trouble with).

Your explanation is probably more lore accurate, but to me it's less elegant and more offensive than 'the oppressive government with questionable morals and their not-so-good founder decided to indoctrinate their most powerful warriors to identify as male because sexism'. But YMMV, I just never liked the 'process only works with men' explanation.

And before anybody even thinks about it, Sisters of Battle are not female equivalents to Space Marines except in the loosest sense, they're not even augmented.

And to be fair, most Emperor's Children have probably left our ideas of 'sex' and 'gender' behind sometime before they began painting their armour a mixture of hot pink, neon green, and florescant orange.