Joey: How you doin'?
Chandler: If I were a guy, and...did I just say IF I were a guy?
Rachel Green: Oh my God, I'm Kip.
Joey Tribianni: You're not Kip.
Rachel Green: Do you even know who Kip is?
Joey Tribianni: Who cares? You're Rachel. Who's Kip?
Chandler Bing: Kip was my old roommate.
Joey Tribianni: Oh, THAT poor bastard.
Answer: The statements that she's contradicting (that it is common, it happens to every guy, and it's not a big deal), are the things that a woman commonly says to a man who is suffering from erectile difficulty, typically to assuage his bruised ego. However, most men do not believe that these statements are true, as evidenced by Chandler's outburst. He's so caught up in the proof that women are lying about it that he gives himself away.
Rooster of Doom