John had said, and I believe it's true, that he was speaking about all of the affairs he had that his wife didn't know about.
I think he felt guilty and wanted to confess, but knew he could not, so he did the next best thing, wrote a song about it!
As it is, the phrase indicates that the words "the fire" refer to a specific fire that the listener would already know about in this house or apartment, such as what would be in a fireplace.
The Beetles did have a sardonic relationship with their fans, so Paul's sarcastic comment that it was about John torching the place of a girl refused him was clearly hyperbole about John's skill in bedding women. Someone who can essentially seduce anyone he wants isn't going to be so upset at a frustrating evening that he torches a woman's home for not putting out. What's really important is to listen to the lyrics as sung, not as assumed.
Seriously everytime I hear this song I think of playing pool at one of my friends houses, great never going to be able to hear the great song without thinking of that now, lovely, but I really do like this song it's very clasic Lennon.
Just wanted to add my interpretation of John's beautiful masterpiece, Norwegian Wood (from a female perspective).
The idea that the lyrics are "So, I lit a fire..." is clearly just an urban myth--the carelessness of an early transcriber becoming the accepted reality for all eternity. I the magazine Rolling Stones, "100 greatest Beatles songs", among the first 20 songs, Lennon has 11 or 12, !