Post-dungeon, the flower thing is a second flower picking which is the actual proposal which only happens after the quest is done. There was no need to reload the save, because the flower at the start of the dungeon has no actual impact. Also the line of dialogue right before the pre-dungeon flower is basically just "in our culture, we use those flowers to propose" so I'm not sure how you missed that until after the dungeon, but again, the pre-dungeon flower does nothing.All I know is I did the "Harboring a Fugitive" quest awhile ago, hit the point where it said picking a flower from the cave was a marriage proposal for Ahmri, wasn't into that, so I re-loaded a save from before the quest, re-did the quest, specifically did not pick the flower, and freed the centaur village.
That said I do understand the desire to not want to marry her. Though I'm fairly certain that there's a choice you can pick when interacting with Ahmri when you enter her room post-quest which is basically just "woah, hold on a second, I ain't looking for this". I'm not totally sure if it's just "I'm not looking for sex right now" or "I do not have interest in marrying you," though, as I've never picked it.