By the time that's happened you've actually already formed a relationship with her. The Serana character was more than a simple, mindless bot, she had a fully fleshed out backstory that allowed for an emotional attachment to be formed. I really enjoyed Dawnguard, I actually think the story was a hell of a lot more interesting than the main campaign but I'm baffled as to why the logical extension of marrying Serana wasn't just not included, but that Beth actually went out of their way to prevent.
Do you think there is some kind of a moral code Beth. seems to be sticking to, vampires = evil therefore can't unlive happily ever after, and kids that can't be harmed.
I actually didn't like the story, just too many holes in it for my liking.
Not sure that the Bethesda vision of marriage in Skyrim matches most peoples expectations and that you gain anything worthwhile by it
I find it falls rather short in the nuptials department but then I could just be a case of loss of dragonbone.
You need a PC and ASX spells - more of that kind of thing than you can poke a bone at, It well and trully compensates for Beths. lack luster approach and you don't have to bother getting married.
The marrage system in Skyrim is lame and I don't think they will do anything to change it either, can't see them going to the trouble of extra voice acting now.