The Dora Milaje as "wives in training" was more of a thing in the previous BP series, where it was kind of a plot point. Basically, the Dora Milaje were part of a tradition intended to keep the various tribes of Wakanda united, by having each tribe connected to the Black Panther. They were considered "Wives in training", though the Black Panther was not obligated to marry any of them (they were unable to marry until released from service, however). They were also much younger than T'Challa, leading him (Priest BP seemed to be in his late 30's/early 40s) to see them more as wards than potential girlfriends. This became somewhat of a problem because one of his Dora Milaje was infatuated with T'Challa, and frustrated by his love for Monica. After Mephisto manipulated T'Challa into accidently kissing this Dora Milaje, her feelings became stronger, eventually leading her to attempt to kill Monica. Despite her attempts to cover up what she had done, T'Challa immediately saw through her lies and fired her. She eventually ran away, and became a villain in the Priest series.
Keep in mind, I don't think most of this info has any pertinence to the current series, where T'Challa is younger, the Dora Milaje are older, and appear to be strictly elite bodyguards.