I finished the DG quest line and I know that Serana is supposed to not be marriageable. However I wanted to see the dialog for myself, but I can't seem to get it. I know she likes me (I was nice at every option) and I can cure her (tried it but reloaded as I want to keep her a vamp). Does she have to be cured first? I have talked to the priest and have a Mara amulet. However wearing it and talking to her doe not get me any new dialog whether she's a follower or not.

Is there something else that needs to be done before the option comes up for her?

Thanks.

I think that even after you cure her you will still not be able to marry her, unfortunately.

I know I can't, but I read that she says something about it (rejects you) and I can't seem to get that dialog.

Hmmmm. Wearing the Amulet of Mara should bring it up.... Why not just reload an older save you may have from after you finished the MAin Quest, and see if she gives you the dialog option...

From what I read, it's only if you sided with the Vampires. It's what you get as an alternative to the cure dialogue on the Dawnguard side.

That might explain it. I sided with the DG on this play through. Next one will be with the vamps.

It does not matter who did you sided with. It's a dialogue check. Something about how she is lonely. Then she asks you about your family. You should always take dialogue options like "are you still feeling lonely?" , "I am glad to be with you" etc. And the wiki had a bad info. You can trigger it before going to Soul Cairn.

This is correct and I am wrong. It goes (after checking):

You: Tell me about your family
Serana: blah blah blah
You: Were you close with them
Serana: blah blah blah
Serana: What about you, what were they like?
You: I never knew them, I grew up alone
Serana: blah blah blah
You: Do you still feel lonely
Serana: blah blah blah
......
You: You talk about being lonely a lot
Serana: blah blah blah
You: Do you still feel lonely
Serana: blah blah blah
You: You mean do I get lonely?
Serana: blah blah blah
You: I try not to rely on others
Serana: Does it bother you I am here
You: I'm glade you are with me
Serana: Me too
....

Wear Amulet of Mara and you get the rejection

Thanks for the info. But it needs a slight correction: Serana: blah, blah, emo, blah, emo, blah