Lets see here for the case for Serana marriage.
From a RP point of view:
You first wander until you reach a guy who asks you if you want to join the Dawnguard, a vampire hunter group. You then make the LONG ( in Skyrim time) to Riften and then take the dangerous path filled with bears until you finally find a crevice. You then walk-in having a feeling to find the dawnguard. They give you a crossbow and some bolts then ask you to investigate dimhollow crypt. They ask you because the hall of the vigilant was ransacked. You walk in be greeted by a few vampires and death hounds. You then fight a master vampire and a named vampire. Lets see, the vampires were too stupid know how to unlock the chamber so you unlocked it. You see a mysterious woman with an elder scroll named Serana, who looks like a vampire. Instead of making her eat bolt or her attacking you, you decide to talk. After she figures out she is a vampire and you are human, she doesn't attack. She then asks you take her to her Castle which is half way across skyrim. She decides to spare your life and not drink your blood when she is home. Your character now chooses to be a vampire lord or stick with the dawnguard. If your character chooses to be a Vampire Lord, she still decides to adventure with you. If you side with the dawnguard she decides to find you and leave her castle (she doesn't know of the prophecy yet). She then asks you find her mother to make sense of everything. While trying to find her mother, you have talks about your history and her history and if she enjoys being a vampire. When she finds out that she is the key for the prophecy, she still sticks with you rather than leaving skyrim or hiding with her mother in Soul Cairn. She then comes with you in a falmer infested place and battles a snow elf with you. She then tells you that her father has to die.
vs
You get a some fire wood for some girl you just meet and sell it to her for a proposal.
Hints that you and Serana like each other:
- You/she betrays the dawnguard/vampires.
- When seeing a scenic place, she says she is glad to be with you.
- She is disappointed when you tell her you are married.
-For heck sakes she gives up her vampirism for you
- She bites you on demand to make you a vampire
- She gives you arrows with her blood on demand
- She tolerates the crap from the Dawnguard
You're sweet. And... I'm not stupid. I can see what you're getting at. But... that's just not something I'm going to be able to do. Ever. Given how I've lived... what I've done... I can't ask the gods for a blessing like that."
That's what you get if you tell her "No. I was just curious." when she asks about the amulet.
There's the unused
"Look, I care about you. A lot. And I'll come with you wherever you want to go. But... with everything in my past, I don't think I could bring myself to go into a temple and ask for that kind of blessing. It's just... it's not for me."
Don't tell me that. She lives with people who want to kill her kind, comes to temples with you and even attends you marriage if you get married. She is not afraid of temples. Even then getting married at the temple should be an option if you cured her or getting married at the blood fountain in the vampire castle if your both vampires and siding with the vampires.
Unless Serana is going to be relevant again another DLC (Which I would pay good money for), there is no reason for such emotional attachment and character development with the dragonborn with an epic rejection at the end.
No other character, I mean no other character has such character development. Aela doesn't even come close but is in 2nd place IMHO. And would it also be too much to ask to still discuss about each other's past after fishing the main quest rather than being generic 99% of the time.
Yes I'm working on a mod but its frustrating that Bethesda would just play with the emotions of people with any word about it.