The One With Joey's Award - S7-E18
Trivia: The original script for this featured a subplot with Ross seeing Chandler exiting a male strip club, with obvious confusion and assumptions, with Chandler later admitting he only goes there because he likes the sandwiches. Matthew Perry shut the idea down. He's barely in this episode, possibly as a result of this subplot being excised and it being simpler to expand the existing scenes rather than write a whole new Chandler-centric section.
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