Actually, the song is not about desire at all. It is about the end of a relationship. John Mayer is using a metaphor, yes. But the metaphor means that, even though they were once truly in love, they are now in the last tender moments ("slow dancing") left in a relationship that is doomed to end ("a burning room"). It's a picture of two people trying to enjoy whatever romance is left between them, knowing that most parts of the relationship are ruined and will soon close in and ruin even the romance (i.e., the fire burning down the room -- the relationship -- will soon burn up even their few romantic moments, their "slow dance").

Hello Bean, It is figurative. Their hot sensation filled the room and as a burning fire needs to be extinguished so does their desire. It is an exaggeration to refer to their great desire. From the song's lyrics he tells her: "baby, you're the only light I ever saw". "burning" could also mean lighted up as by fire or flame ,so it is her light he sees lighting up the whole room as if it is burned. It is in all cases a metaphor.