If I did have a tumor, I'd name it Marla.

"I haven't been fucked like that since grade school." blew my young mind, at the time.

https://www.imdb.com/title/tt0137523/trivia/?item=tr0755880
The original "pillow talk"-scene had Marla saying "I want to have your abortion". When this was objected to by Fox 2000 Pictures President of Production Laura Ziskin, David Fincher said he would change it on the proviso that the new line couldn't be cut. Ziskin agreed and Fincher wrote the replacement line, "I haven't been fucked like that since grade school". When Ziskin saw the new line, she was even more outraged and asked for the original line to be put back, but, as per their deal, Fincher refused.
Also, the actress didn't know grade school in the US is 6-12 years old as she is from outside the US. She didn't find out until afterwards and was very unhappy about saying it.
- Fair point.
  When I saw the flick, the internet was barely more than geocities & altavista, so 🤷🏼‍♂️
  TMI: as someone who lost theirs at 12, I guess it resonated with me — but, I completely hear where she's coming from.
- why? she's an actress ffs
Helena Bonham-Carter, being British, didn’t know that “grade school” was much younger than one would expect from such a line until after the film was released.
- They kept it in because it isn't inaccurate. 🤢🥲
  coughpriestsarentafraidcough
I think the original line was:
"I want to have your abortion"
But it was cut because it was deemed to krass.
- crass*, FFS. (and too*, while I'm at it)
  If by "cut", you mean "not included", then that's a whole other can of worms re: your comprehension of this shared reality we live in.

Nah, Marla was real. PM members physically brought her to Jack at the end. Even PM wouldn't be able to pull off bringing an alter ego to Jack, she must be real.

Also, in the film the thrift store cashier LOOKS at Marla during the conversation.

She's probably real in the novel as well, but I don't think you can definitively say it's an impossible interpretation in either.
It's a bit more clear in the novel for a few reasons but you can still wave away a lot of the points against it simply by pointing to how extremely unreliable the Narrator is.

I watched fight club for the first time in 10 years recently. I’ll tell you that movie hits different when you’re 20, 30 or 40. I remember in my 20s it all seemed so cool and badass, now that I’m 40 I see now that they’re a bunch of incel losers with toxic masculinity and a lack of self awareness.

They are a bunch of losers, but their plights are very legitimate. That's how a charismatic cult leader swoops in and radicalises them.
We have a number of real world examples of that. The most current one being Andrew Tate.
The point was to not worship these people. The point was driven better in the book, the movie was more anti-capitalist than the book.
Chuck P was a gay man. The movie takes the homoeroticism to levels that would have been laughable if Palahnuik had written them.
Why does the narrator ignore Marla? BECAUSE HES GAY