What is known of Space Marines gene-seed transformation are the following:
- the Emperor, whether lying or truthful, states that only men can be Space Marines; so, while this does include transwomen, it does not include women or intersex people;
- whatever the circumstance, scientific progress by the scientific method and empiricism would be deemed heretical due the Imperium/Mechanicus's obsession with STC technology; so, if one wants to mess around with the Emperor's line of experiments, it would have to be done very discreetly, which would still probably be noticed by Inquisition, so the only way around that is to be part of the Inquisition. Or you could try to take deep into Mars in the old parts of the Tech-Priest civilization, which also comes with its own problems.
As Space Marines, Thunder Warriors, and Custodes are not all the same thing, and the Emperor specifically was talking about Space Marines, then it can be assumed that Thunder Warriors and Custodes (the most powerful of the three), can include women and intersex people.
It should be noted this is quite literally the only thing women can't do in the Imperium. They can join any other branch of the military and attain any government or order position or anything in the professional world besides being a Space Marine and all humans of the Imperium are citizens holding equal rights.
This is assuming of course that aspect of Imperial law, of equality, is actually enforced throughout all the planets of the Imperium, but I suspect that where the Adeptus Sororitas,the Adeptus Mechanicus and the better groups of the Imperial Guard, Knights, Ordo Tempestus, Imperial Navy, Adeptus Astartes and the Arbites are, it'll be enforced. I put the Tech-Priests up there because they would find sex and gender equality illogical, so they have that to be thanked for despite their other shortcomings in human right.
