The One With All The Rugby - S4-E15
Stupidity: Emily tries to help Ross inflict pain on the other rugby players and tells him about one player who doesn't wear a cup. No one who knows rugby would ever wear a cup, it is more hazardous than helpful to wear a cup in rugby as it involves a lot a grinding, and a simple slip or movement could cause serious damages to the male genitals. Especially if these guys are British and well versed in rugby which they are.
The One With All The Resolutions - S5-E11
Stupidity: When Ross is in the bathroom, there is literally a towel right next to him and he doesn't use it.
The One With Frank Jr. - S3-E5
Stupidity: The friends discover that Joey's built a ridiculously oversized entertainment center only after they've pushed it upright. Cute concept, but there's no way that's when it would have become obvious. First of all, all they had to do was see it stretched across the doorways when it was lying flat on the floor. Second of all, Chandler had been watching it get built; even if he wasn't really paying attention, it would have been very immediately obvious at a single glance. And Joey's a space cadet, but there's no way he wouldn't have noticed either.
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