See, to me, who the characters are, and who they are romantically interested in, are not easily separated in my mind. Only if the relationship is badly written or handled, do I start looking for alternatives. And while it is theoretically possible for Zuko/Katara to be done well, it really generally isn't, and I've no desire to sift for the golden fic in all the sewage.

The common theory is that the fangirls projected themselves onto Katara, and since they wanted Zuko and thought Aang was a baby, so did Katara. I can't disagree with that, not by much anyway.