Answer

It could mean when they dance the narrator is dancing one way (her obsession for Marilyn) and Marilyn is dancing a different way (she feels nothing but friendship for the narrator and is not interested in her in any other way).

It could be the narrator has a huge, obsession-like crush on Marilyn and it is as if this is ingrained in her since before birth. This ‘glamorous love lasts …’ shows that her love is strong and it will last through time, even ‘before you were mine’