I can think of no better way to put it...
Ross: Chandler was totally flirting with the hot delivery girl!
Chandler: Thank you for that! (To Monica) I was not flirting.
Monica: It's okay. I don't care. It's fine.
Ross & Chandler together: Really?!
Monica: It's no big deal, I do it all the time.
Chandler starts to laugh, but then gets serious: So umm, you-you flirt with guys all the time?
Monica: Sure! It doesn't mean anything! Just like I know it doesn't mean anything with you!
Chandler: Okay, but there is a big difference. You are a lot hotter than I am.
Joey (walking past): True story!
Later...
Monica: Okay, let me get this straight, it's okay for you to flirt, but not for me.
Chandler: Oh, I'm so glad we cleared that up. Look, I'm sorry, some things are different for men and for women.
Monica: Go on, teach me something about men and women.
Chandler: Okay, I've already taught you so much already, but whatever. See when you flirt with a guy you think, "I'm just flirting, no big deal". But the guy is thinking, "Finally! Somebody who wants to sleep with me!"
Monica: No way!
Chandler: It's true.
Monica: Well that's pathetic!
Chandler: Again, true.
Monica: And this goes for all guys?
Chandler: All guys that are awake. Then we go to sleep and then all the guys from the other end of the world wake up and behave the exact same way.