The One Where Rachel Tells... - S8-E3
Easter egg: At the end of this episode on the DVD there is an alternative story for Monica & Chandler in the airport going on honeymoon. The story is about Chandler making a joke about a bomb in the airport. It was due to air not long after 9/11 so they changed the story in respect of that.
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