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.
You can wear your power armor anywhere you want. Anyone who would object to your power armor also probably can't actually do anything about you wearing power armor.