One of the more popular marriage requests is that of Serana, many gamers wish to marry the central vampire companion from the Dawngaurd DLC however, she and any other vampires placed in the game via this expansion pack are actually unmarriable via the in-game mechanics. This decision was so heavily disagreed with, that the modding community of Skyrim decided to make it right. Therefore, to marry Sarena one simply has to download the "Marriable Serana" mod by C0drm0nk33 from the Skyrim Mod Nexus. After properly installed, Serana will now be moved into the 'marriageable faction' in your playthrough.  That being said, you still have to go through the regular procedures via the Temple of Mara as well as completing the main questline of the Dawngaurd expansion pack.