What does " The Flintstones could've really happened"?

[Joeyy]

Chandler: You know, I can't believe you. Linda is so great! Why won't you go out with her again?
Ross: I don't know.
Chandler: Is this still about her whole 'The Flintstones could've really happened' thing?

---Friends S01E18

 

[bubbha]

The Flintstones is the name of a well-known cartoon series. It's set in a fictional prehistoric era in which humans use Stone Age resources to live like modern humans.

Apparently Linda believes that the fictional world of The Flintstones might have been possible, which makes her seem eccentric or crazy, which was a turn-off to Ross.