Players cannot marry Serana in "Skyrim". Although a variety of female nonplayer characters are available for players to marry in the game, the vampire in the "Dawnguard" expansion is not one of them.
During the quest Chasing Echoes, players have the ability to ask Serana to marry them, but she always refuses, making claims that temples make her uneasy and mentioning something about a complicated history. Despite this fact, she follows players into temples during the course of the game. She cannot be married even if she is cured of her vampirism.
Though she cannot be married, she can be recruited as a follower to assist in the challenges that the player faces over the course of the game.