"Well, I'm hard enough to pound nails right now," Duo replied in a funny voice.

Heero blinked, wondering why one would want to. "You would scratch your skin."

... I'm sorry, I couldn't help it. I'm extraordinarily stupid. Why does Heero say 'you would scratch your skin?' Heero thinks that Duo is saying that he could pound nails in with his cock, and he wonders why anyone would want to... but what does that have to do with scratching your skin?
I really hope I'm not being too annoying with such a stupid question!

P.S. *_____* I really really like how Heero and Duo are characterized here!