The One with George Stephanopoulos - S1-E4
Trivia: When the girls are out on the balcony discussing what the boys would be like in bed, their discussion foreshadows their future romantic entanglements. Phoebe speaks highly of Joey, Monica of Chandler, and Rachel of Ross. Although Phoebe and Joey were never technically together, they often acted as a pair.
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