You must complete the questline in order to ask her to marry you, but she politely refuses, saying that you're great and all but it is not meant the Dragonborn and her to be married due to her uneasy history and she does not like temples.
I was hoping to listen to that dialogue line and I tried talking to her after every DLC quest. Even after wiping out the DG, I never got that line.
Also, I didn't get the dialogue line if you try to give the bow to Harkon and Sarana says that it is an idiotic decision, even though my friend did.
Maybe some of her dialogue is bugged?