I'm going to refrain from telling you "it's a fantasy game, just roll with it" and try to talk from an in-universe perspective.
I think there are far worse problems with WoW's storyline than Jaina dating a dragon who lives in human form. Remember Kalecgos' previous girlfriend was a font of magical energy shaped into a human form by a red dragon. Literally anything goes for that guy but before you judge him for it also remember the blue dragons are almost all gone now, the blue dragonflight was disbanded and they all went their separate ways. There's no online dating service in Azeroth, you take what you can get. Then again I've always thought interracial fantasy couples (beyond the boring old elf + human one I mean) are kind of cool plus Jaina now has a boyfriend who can turn into a dragon, that's pretty cool.