The One Where Ross Got High - S6-E9
Rachel: You guys, it was bananas, cream, and beef! I just cannot believe that you ate that so that I wouldn't feel bad.
Monica: Actually, I didn't eat mine. It's still in the bathroom.
Joey: No, it isn't. I ate that.
Judy: And we left ours in Monica's bedroom.
Joey: No. Got it, [nods to Jack] and got yours, too.
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