addie wants to marry mary, but realizes he needs court rank for it. he then tries to consult their friends about it, and when stumped finally they approached mary to have her ask her father to give addie a court rank because 'there's something addie wants to do that won't be possible without court rank'. mary, not knowing what addie wants to actually do, is like "if you really need court rank, you can just marry me" and everyone goes "....."
plus mary herself doesn't really mind marrying addie? by the time addie has to prepare for the marriage (administration papers and stuff) mary was busy being in boarding school in the next country, and basically hands all preparation to addie with Patrick's help. and she's super casual and non-chalant about it. even addie goes like "are you really okay with this? at this rate you'll really be married to me" and she's like "??? yes, that's the plan, right?"