I just watched The Cave of Two Lovers. Is it a coincdence that the writers wrote about two lovers?
As you watch the episode, Katara's feelings for Aang becomes slightly clearer. They didn't have to kiss. You can tell through actions.
Evidence 1: Katara is getting upset over what Aang is saying. If she didn't have feelings for him, she wouldn't have become so angry (Katara would have gotten slightly annoyed, but not angry).
Evidence 2: Katara blushes at the end of the episode. There was no other reason for her to blush, so it could mean that she is blushing about what happened in the cave.
Freebee 1: While I doubt that there was a kiss in CoTL, I still want to break a favorite Zutara argument. After the torch goes out, there is a few seconds of darkness before the crysals started glowing. I've heard someone say that the crystals started to glow directly after the torch goes out. This is not the case. Watch the episode for yourself, there is at least 3 seconds of darkness.
All that I've stated is not direct proof of Katara's feelings, but it does shine a light on that possibilty. In fact, it makes the possibility probable. Go Kataang!
(I do not wish to get into an argument, although I will debate. )