Who guys are in fight club is not who they are in the real world. (6.16)
Fight club allows these men to wear a different hat (if hats were allowed in fight club, that is) than they do in real life. The guy who washes cars by day can be the king of the ring at night. Fight club: the great equalizer.
I am Jane's Uterus. I am Joe's Prostate. (7.19-7.20)
This recurring gag in the book also serves a pretty important thematic purpose. It shows our narrator identifying himself not as a human, but simply as an organ, a piece of a larger whole that isn't totally in control.
"Sticking feathers up your butt [...] does not make you a chicken." (8.71)
Good to know. There is a point here, though: you can't just talk the talk, you have to walk the walk. Or fight the fight, as the case may be.