One kiss, when there is no love and nothing else further occurs, isn’t cheating, it just makes the person kissing someone besides their partner an asshole who is showing the potential of what they might be capable of later. I think anything further than that is cheating.
I also believe in emotional cheating, wherein there may not be contact yet but where love or emotions or intent is involved… that’s TOTALLY cheating! I think the emotional betrayal is deeper than the physical.