Installing Android Market/Google Play on non-rooted Kindle Fire
The necessary .apk files for Google Play Store, Services Framework, etc., must be installed in /system/app and doing so requires root access. So, you must root the Kindle Fire in order to do so.

However, you don't have to leave the Kindle Fire rooted. After you've installed it, you can always unroot the device (though I'm not sure why you'd want to do that).