Yes, yes they did. Good points all, on your part. Spot on about players wanting the DB marrying her and all. I think at that point, she was so fresh, new, and a vampire for cryin' out loud, that only maximized her attractiveness as a spouse. Well, that and perhaps the fact that most were probably tired of their current spouse or even bored with what else there was to choose from.
Serana? A breath of fresh air, in a manner of speaking, since y'know, she doesn't really breathe.
Couldn't have said it better myself. With the exception of the Companions (as in, Aela, Vilkas and Farkas), Brelyna, Camilla and Ysolda, I don't think there are really any very interesting choices to marry your DB to, regardless of gender. There's Muiri, if you want a "dark marriage" (because let's be honest, she'd try to have a way to kill you in your sleep if she thought you double-crossed her in any way) for your assassin, but if you're not an assassin/mage/Companion, what else is there to choose from? Literally Ysolda and Camilla. Those are the only ones. Even Mjoll and Aela/Vilkas/Farkas aren't options I'd pick, even though I like their characters, because marrying them means confining them to a "domestic" (for lack of a better word) life whenever they're not adventuring with you and that sort of takes away their badassery IMO. (Another reason for me to prefer Serana being unmarriable.) There aren't even marriable Khajiit; I mean, come on...
Serana, on the other hand, has a personality. She feels alive (no pun intended), real, someone who the actual player cares for as opposed to only their character caring for in a RP backstory. Now, that's all well and good, because this is a roleplaying game after all, and you're essentially "controlling" someone else, so of course maybe your character would be attracted to/like someone you, particularly, would find bland. But let's face it, a lot of players want their character to marry someone they, themselves, actually care for. And Serana's relationship with the DB is
shown to grow. It's not out of the blue like most of the other marriage options. That bit just before [SPOILER, I GUESS?]
you enter the Soul Cairn (if you're not a vampire)
[END OF SPOILER]
shows just this. They went from casual acquaintances to very good friends, at the least. That, I think, is what draws people to her. She is likeable. And she definitely likes you, no matter how you treat her. Serana is, I think, excellent writing on Bethesda's part. And I understand their decision of not letting players marry her.
Am I ranting? Please someone tell me if I happen to be (and apologies in advance if that's the case.)