it sounds like this song is about a relationship that was full of sins. you sin - you leave the garden of Eden. the Garden of Eden is where Adam and Eve roamed until Adam sinned and ate the fruit of the tree, then was kicked out for his sin. so when she says "Are we leaving this garden of Eden?" she is saying "have we sinned and must be punished?". or at least this is what i believe, i am not good at this kind of stuff.