User blog comment:DarkSaiyanMitsubi/Serana Marriageable/@comment-1992639-20120817000006

Serana cannot be married, this is intentional. She used to be a worshipper of Molag Bal and probably did some pretty bad things in her youth. When the dialogue of asking Serana to marry the Dragonborn is read well, one can conclude that Serana believes she has no right to ask the divines for a blessing of marriage after all the bad things she has done in the past, even if she has abandoned the ways of her family, Molag Bal and vampirism.

When checking the game files with creation kit, it shows that there is NO option to marry Serana, with all possible answers varying from being outright disgusted when proposed with marriage, to being very flattered and heart warmed, but still refusing out of guilt for who she was.