Isn't it weird to say "I don't" as a respond to "Haven't you ~?" question?
Yes, it is a little odd to respond "I don't" to "haven't you." I believe he says "I don't" as a way to show he is flustered and unsure of how to respond to the question. This seems to be a mistake made by the character because he is being accused of not having kissed anyone and isn't sure how to respond that that.
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