I can imagine this during the finishing touches of the game:
"And so, after Serana becomes your companion, she can marry you after a heart fell scene. Considering we made her one of the most developed characters in Skyrim, I think the players who've grown to like her would appreciate it!"
Todd: "...You know that line about, 'it's not you, it's me' that we put in case the players tried to propose to her before the last quest in Dawnguard?"
"Yes..."
Todd: "Yeah...let's make that the line that Serana says if you propose to her after you finish the quest line. It'll be hilarious! The forums would be filled and we could be like: heh, u mad bro? We can be the trolls for once!"
Good. Except the part where Todd said what I highlighted in bold.
"For once" is an understatement.