So I get a lot of people trying to defend the kiss with her supposedly not understanding what kissing is or whatever, but that definitely isn't true. Chapter 9 has her understanding that kissing is intimate (she refers to it as courting, or the act to gain another's affection) from the books that she read, and her almost violent response suggests some understanding of how Lily feels about possibly not being Heath's first.
Even if she was curious about the "'sweet' feeling" being only when she kisses Lily, she would have enough knowledge to know this wasn't okay. There's no way to spin that kiss being acceptable.