I imagine she might be in the Dawnguard because they're the ragtagiest bunch of misfits in Skyrim.

Can't marry Serana.

Still a better love story than Twilight.

Stockholm Syndrome.

QFT. & it's funny.

I actually tend to assume that her making you a vampire is as close to marriage as you're going to get and a suitable "end" to the romance.

That's a really low bar!

I agree. Most of the DG treat her with indifference at worst. Isran mostly treats her with outright suspicion, which is at least straightforward and free of the political machinations her parents have always subjected her to. In the end, even he admits that she's been an asset, which is more than she got from either parent.