The kiss, while it didn't startle her, did make her take pause. Instantly that all too familiar internal struggle of hers returned. On one hand, Katara couldn't deny how attracted she was to Zuko. There were moments, when they were together like this, that she could easily forget about all of the horrible things he'd done and simply enjoy the fact that they had some sort of bond that just couldn't be ignored.
On the other hand, however, those moments were so few and far between that the blissful ignorance never lasted long. Her hormones could only carry her so far, really, and then her brain demanded to once more be in control. Especially now that she'd left the Hyperion and still had the bitter taste of almost-betrayal on her tongue at the knowledge that Zuko had a girlfriend.
Girlfriend. The one word was enough to make her pull away after returning the tender kiss with one of her own for a good thirty seconds. Blinking, blue eyes wide, she focused on Zuko and tried in vain to fight the flush that was creeping up her cheeks.
"We can't do this," she said quietly, voice strangely lacking its righteous anger that typically followed. "It isn't right. You have a girlfriend."
And, much to her own surprise, Katara realized that was ultimately the only reason she wanted to stop. Her concern that he might turn on them was so miniscule it no longer plagued her in any real fashion. Her fear that he was using her to get close to Aang was all but abated. All that remained was Mikaela Banes, and the fact that Katara couldn't allow herself to come between them in such a way.
And thanks to this sudden revelation, Katara was, for the first time in a long time, well and truly stuck on what to say.