I recently watched my Fight Club DVD with the girl I am dating. She had never seen it before. So when I showed up at her apartment, I told her I had brought her a nice, romantic movie. I was joking, but there is some truth to it.
I actually did not even realize this fact either until I bought a copy of the Fight Club 2004 Literary Edition that has a new introduction by the author Chuck Palahniuk. Near the end of the introduction, Palahniuk mentions all the different genres that the critics assigned to his fist novel, and he ends the paragraph with his characteristically despondent sarcasm, "No one called it a romance." (p.xviii)
The Narrator hated his life. He hated his need for material possessions and all his "stuff" that he had acquired at the expense of his spirit. The Narrator thought that his life was meaningless and he did not want to go on.
"I am helpless. I am stupid, and all I do is want and need things."
-Fight Club (p.138)
Then he met this girl who was free from the "suffering" of materialism, in Buddhist philosophy sense. This led him to a confused state of both wanting her or loving her and wanting to be free from all desire for anything. So his mind created Tyler.
Tyler could do anything. He could have Marla and still be emotionally free from her and everything else. This caused Marla to fall in love with the Narrator while Tyler ran off to help the whole world "hit bottom" so he could rebuild the world free from materialism. It is a butterfly effect-type of situation, everything in the book happens because of the Narrator meeting Marla, but it is easy to forget that once you get wrapped up in the story.
"I know all of this: the gun, the anarchy, the explosion is really about Marla Singer."
-Fight Club (p.4)
So Fight Club is a love story because nothing would have happened if the Narrator had not met Marla.