I admit I tend to feel the same way about Aang/Katara--and I don't ship Zuko/Katara either. I think the fact that Aang looks so young is a really big part of it. Maybe I'm projecting my own feelings onto Katara, but I just don't follow her feelings in this, so I wind up just having to see it as a plot thing. Zuko/Mai works for me since they are at least an obvious pairing given their ages. I mean, there's not that many teenaged girls to choose from and one of the fire nation girls is an obvious choice. For the same reason I'm not surprised when Ty Lee has her little interest in Sokka or Toph gets a crush on Sokka. Or even when Aang starts crushing on Katara. Or Katara likes Jet!

It's just about feeling real, like you said, and maybe not being aware of plot-machinations when you're reading it. It's not like even in a romance you love you don't know that you're reading fiction. Pride & Prejudice is just as plotted as anything else. But you want something to feel inevitable for a reason other than it's where the author is going, even if it's a subplot. Which doesn't mean you need emotional reasons for somebody to like somebody else. But if it's physical you have to believe the physical.