What if at some point before this Killian already asked her to marry him? In a big romantic setting with candles etc, but Emma panicked and said no and didn't want things to change between them etc.. Then after their adventures (separation?) during the finale, something makes her change her mind (as it always does during finales), and this is telling him that she WILL marry him. It would explain why it almost has an organic, unplanned feel to it. Just a thought... Of many in my CS buzzed mind right now.