Serana doesn't want a romantic relationship and she has made it clear that she does not want to get married, so I will respect that and I will be her good friend while I look elsewhere for marriage. My good friend Aurlyn Danwstone brought up the topic of marriage and I like her a lot so I might as well see where that leads, while remaining good friends with Serana.
What if Serana was just saying that she doesn't want to get married for the moment, and when she is cured of vampirism, she feels like she can reconnect with mara again. and what if she was just being indirect, people don't always say what they mean and that applies to no topic moreso than matters of the heart. I'll remain single and wait and see what she decides and keep myself open for her.