This is quite simply a love song from the one who is feeling rather strongly that this love will last. For how long? Well, this writer uses a symbol that survives from ancient times: the eternal flame which, either naturally occurring or manmade, serves as a constant reminder or commemoration of something profound. In the United States, the first eternal flame erected for an individual was for President John F. Kennedy, who had recently been assassinated. So the question is, does this work as a metaphor? If one just looks at the object and not at the meanings ascribed, I believe the answer is yes. The singer seeks reassurance from her lover that is in fact eternal flame (it's always put as a question), but her sustained high note on the word "flame" pretty much clinches it.