Do you mean offensive sites in general or just these file sharing sites?
For torrent sites I think I'd be pissed. I'm no fan of piracy and I'm not an apologist for those sites at all but I don't think it's Google's place to do that. Google's mission is to index and organize the world's information and blacklisting websites like that would be hypocrisy. Even if we all agreed it was okay for one site it becomes a slippery slope where Google can use that for their own or their partners' benefit. There'd be an exodus and Google would slowly lose market share from that point on.
It's one thing to stop making it so easy to find a site that you barely have to type 2 letters before you get what you want. Its an entirely different thing to limit access to certain sites. If its not on Google then it doesn't exist on the Internet for all intents and purposes. We're back to emailing each other links in that case.
This should be as far as Google reaches on this short of banning these sites. From there it's the job of users to stop supporting piracy and law enforcement to shut down illegal sites in their own jurisdictions using the current laws and without creating new laws to prosecute people unless it's absolutely positively necessary.
It's interesting you point out it would be a "slippery slope" to be on.
It isn't a hard stretch to imagine in the future, you need to turn safeSearch off to see them in search results. Sometime further in the future you'll need to add some other preference or something to see them. Then, you'll need to be logged in and have a "verified" account. Then....
I think it's safe to say Google are well and truly on the slope of slipperiness.
To keep the discussion going, how would you feel if in the future, Google removed the "offending" sites from it's index entirely?