the OG space marines are all men not because of in universe sexism but because they are intended by the Emperor to be ultimately disposeable biological war machines, with tightly controlled 'production' dependent on 'baseline humanity'. even the potential to reproduce, or edit themselves to be able to reproduce, would be contrary to this plan. only the Primarchs are meant to lead humanity in any capacity, and even they are meant to fade into obscurity and be disposed of once humanity is in a good position and doesn't need genetically engineered killing machines to protect them from aliens and living emotion monsters.

that being said, after pulling the primaris space marines out of a hat (secret bunker on mars) they could just be like 'wow this machine cult protagonist guy figured out how to make female space marines and due to the recent big warp storm the government can't be bothered to care'

custodes are a different thing however, they are meant to be the emperor's personal companions and unlike the space marines are not meant to be killing machines (even though they are far more formidable in combat than a space marine), they are meant to be intellectuals and philosophers also. they have a lore counterpart in the Sisters of Silence, who are all women but cannot speak due to their anti-psychic abilities being so powerful that their voice would shatter a mortal person's mind (kinda misogynist to make them all silent but they are at least the intellectual equals of custodes and can write/read afaik) but since the custodes and sisters are such personal projects of the emperor (who had a woman scientist help him with all this genetic engineering afaik) they could easily contain among them unique and exceptional individuals.

in conclusion basically the entire point of the 40k setting is that it is so implicitly huge and diverse and detailed that you can plausibly fit a lot of headcanon or new lore into it, there's no real diegetic reason for GW not to introduce female or gender nonconforming space marines or custodes. the emperor is basically dead so he can't stop them, Guilliman is back and allowed Cawl to make the new primaris space marines (such heresy!), and the galaxy and therefore the Imperium is split down the middle by a huge chaos storm with each half led by different guys. Its like the perfect time to introduce non-male space marines and custodes, in fact it would be nice to have non-male models for other factions like the imperial guard, who canonically recruit men and women and even mutants, even the relatively good-guy coded Tau only have the single woman character (Shadowsun) as an actual model you can get.

Sisters of Battle do already exist but they literally have spiky boob-shaped breastplates so idk if i can give them any feminist credit for that.

though given all the crotch-mounted skills and breast spikes in the setting i can only assume that the Imperium isn't as much misogynist as it is entirely sex-negative (it can summon demons due to chaos if you feel too many emotions too strongly)