The One Where Paul's The Man - S6-E22
Trivia: When Joey's trying to get his picture up in the dry cleaners, the owner claims never to have seen Air Force One. The actor playing the owner is Ilia Volokh, and he actually played one of the terrorists in Air Force One.
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