What do you mean "That's not the same thing. I want to be able to marry Serana in the same way I can marry other NPCs!"? Come on, now. Is that really what you want?
Your opinion may differ from mine, but to me, the combined benefit of a daily homecooked meal, the Lover's Comfort bonus, and the convenience of an in-home merchant simply isn't worth what the marriage mechanic does to an NPC--- reducing them to the status of an annoying talking doll that pulls it's own string every time you brush past them. "Yes my love? What do you need?" Nothing, for Christ's sake! Didn't you just ask me that five seconds ago?
Yes, Bethesda has somehow made fantasy marriages even less interesting than real marriages (a feat I didn't think was possible). Just for once, I'd like to be able to go off adventuring away from home for several days, and then arrive home only to discover that my wife has tossed all of my stuff onto the front porch. And that she won't speak to me. And that I'd have to go buy her a gold necklace or something in order for us to return to business as usual. Or heck, toss in a PMS mechanic that raises your wife's aggression level for seven days a month, and has her doing things like throwing pots across the room and yelling at you without provocation.
Marriage in Skyrim is static, repetitive, and gets very dull very fast. So who cares if you can't technically get married to Serana?
Besides which, I don't need an in-game mechanic to dictate what sort of a relationship I have with an NPC. Serena is great in that you can easily pull off imagining a marriage to her. She and I currently reside in Honeyside (Riften). When we're at home I tell her to wait in the kitchen and she remains there (you know, like a good wife should) where she'll alternate between using the cooking pot and chowing on a loaf of bread. When I feel like sitting out on the deck. I'll bring her along with me and she'll sit across from me. Or she'll lean up against the rail and peer out at the water. I can make her wait anywhere in the house where she won't get in my way (depending on what it is I'm doing) and at the very least she'll interact with things and won't just stand there like a golf caddy like other follower/spouses do.
But most importantly, she doesn't yammer my ear off with that same repetitive dialogue which is unavoidable when it comes to "real" Skyrim marriages. Best Skyrim marriage I've had so far.