The One Where Ross Dates a Student - S6-E18
Ross: I would date her, but there is a big age difference.
Joey: Well think about it - when you're 90...
Ross: I know, she'll be 80 and it won't be such a big difference.
Joey: No, that's not what I was going to say at all. What I was going to say is, when you're 90 you'll still have the memory of what it was like to be with a 20-year-old.
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