Some fans of the show "shipped" the character of Katara with another character called Zuko, but in the end she ends up with the show protagonist Aang. There's always been a rift in the fandom between people who supported this decision and people who thought it made more sense for Zuko to end up with Katara. Personally I think either answer works, Zuko and Katara do have a chemistry, and despite the final decision of the writers they do also find a lot of excuses for the two of them to end up going on missions alone together, plus the two are more emotionally mature than Aang who's a bit younger both in age in behavior. But also the writers created another love interest for Zuko who I feel has equal chemistry and while Aang is a bit immature by the end of the show he's come into his own a lot more and this is when he confesses his feeling for Katara.

On top of all this though is the fact a lot of people who were into the Zuko-Katara pairing also pumped out a lot of NSFW material about it. Aang is younger in both appearance and behavior so he didn't really get that treatment as much because I guess people would have gotten pedo vibes from it, and while Katara and Zuko are both still underage they look and act more mature so I guess people felt they could get away with that. The red flags are probably coming from fans who see the writers of this show aging up Katara a bit as a means for them to inject for risque material into the franchise, which leaves a lot of fans feeling... weird. The show proper implied some of the characters may be sexually active (Sokka and Suki for one) but because the main cast is almost entirely underage this was kept on the DL, a lot of people are uncomfortable with it being more overt.