The One With The Truth About London - S7-E16
Visible crew/equipment: As Joey is entering Monica and Chandler's apartment, there is a black microphone visible for a few seconds moving above Joey's head as he is walking towards Monica and Chandler at the table.
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