There's a time and place for everything
Now, let me be perfectly clear: I'm not saying you should ALWAYS eat pizza with a knife and fork. If you're eating a single slice, using a knife and fork is just silly, and asking the guy behind the counter for some cutlery is a good way to get banned for life. Likewise, if you're at a party and someone orders pizza, you should go with the crowd and handle that slice with your bare mitts.
If, however, you're at a restaurant with a few other people, and you've ordered a whole pie that arrives at the table on a metal tray, hot and fresh from a brick oven imported from the old country, you owe it to yourself to try using the knife and fork that were there when you sat down.