Answer:
an unconditioned stimulus
Explanation:
Unconditioned stimulus: In psychology, the term "unconditioned stimulus" is described as one of the different aspects that is being included in the classical conditioning theory and was proposed by Ivan Pavlov. Unconditioned stimulus is also denoted as UCS or US and is considered as a stimulus that tends to elicit a response automatically, unconditionally, or naturally.
In the question above, the long, passionate kiss is an unconditioned stimulus.