Ok, yes they should have made/should make her a marriage option considering all the crap you go through with her, but let's face it: what is the point of marriage in Skyrim beyond the acheivement? You get a specific character in one of your houses who just gives you 100 gold per day and a meal after a rather cheesey marriage ceremony in the horribly trashy hold of Riften. Oh and then they call you love or dear every time you walk by...
That's it? Exactly. Now incorporate the fact that only a handful of all the marriage candidates will actually follow you and aid you in combat. It's really not that big of a deal. Serana already indirectly flirts with you in some of her dialogue, so if you "love" her so much just have her follow you everywhere and just assume she's your lover. It's not that much different, when you think about it.