The One After the Superbowl, part 2 - S2-E13
Chandler: Stick a fork in me, I'm done.
Phoebe: Stick a fork what?
Chandler: You know, like when you're cooking a steak.
Phoebe: Oh, I don't eat meat.
Chandler: Well how do you tell when vegetables are done?
Phoebe: Well you just, you know, you eat them and you can tell.
Chandler: Okay, then eat me, I'm done.
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