The One Where Nana Dies Twice - S1-E8
Trivia: In the "One where Nana dies twice" at the end of the episode the cast looks over pictures of Ross and Monica's Nana. One of the pictures is labeled 'Me and the gang at Java Joe's'. Java Joe's was one of the name possibilities for Central Perk during the show's creation stage.
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