But there are people like me who have both. How would your proposal handle this?

Unless you download that sort of stuff directly, no files like that ever reach your PC. About the only thing that travels to your PC are the searches, which your computer routes to other nodes even if you don't have the files that meet the search criteria. The file transfers themselves are made outside the Gnutella network (using HTTP) via a direct connection between the person requesting the file and the person hosting the file. Your PC has nothing to do with the file transfer between them.

It would be seriously destructive to the Gnutella network for some clients (or users of those clients) to arbitrarily decide that they don't want to forward queries just because they contain the word "porn" or something.

A user blocking another user's queries isn't proving anything. How are you giving others the right to publish what they want if you insist on blocking queries to it? How are you upholding the "rights" of others to search for something by blocking it just because YOU disapprove of it? Remember, all you know about is that they're searching... the transfers happen without your knowledge anyway, so why should you care what they're searching for? Also remember, you nor your PC are taking any part in the actual propogation of that sort of stuff on the network.

I think your idea would be better suited to a proprietary network where there are only 1 or 2 clients that exist to access it. Unless you can convince ALL other Gnutella client developers that your idea just rocks and to implement it (which I doubt would happen... you haven't yet convinced me it's worth implementing into my client), producing a client with features like that would be useless because there would be a dozen other clients out there that would be ignoring your efforts.