Because marriage in Skyrim makes the NPC a robot, so marrying Serana would completely ruin her character, unless they made marriage to her different I don't want to be able to marry her.
Haven't we disscussed this before? I don't think it would be to hard for Beth. that if you could marry her you would be forced to have her permanently as a follwer.
I could stomach that, and those who can't well it would be your fault for marrying her - you know head strong girls and that kind of thing.