I think a very accurate, yet simple metaphor would be something like:
Imagine this: If a man sings a Lady Gaga song in a park, then Universal Music Group will be able to close the park.
(Turn it into: "If a girl sings a Lady Gaga song in her school, then Universal Music Group might ask to close the school." and you may win every mother's support =P )
Or also: If a thief tells someone where he left a stolen wallet, government may close the buidling where the talk took place.
PS: Hi everyone, I'm from Argentina, but I really care about SOPA because it may set a very bad precedent in Internet legislation. Right now, in a lot of countries, this kind of bill would make the proponent look like someone come from Iranian or Chinese government. A fascist person looking for a way to censor the Internet. But if this bill passes, a GLOBAL consequence will be that politicians in other countries (like Argentina, in my case) will say: "Look, this law was approved in the USA. And they care about Freedom of Speach, so, it's not a big deal. -End of story-". And of course, they will use laws like this, to censor much more than just piracy. I dont want to sound fatalistic, but if this bill passes... well, we can start saying goodbye to the Internet as we know it.