Hi,
Basing this on the meta tag is not fail safe, the best way to do this would be to use the plug-in PureSight for WinGate, which is available for download from our website,
http://www.wingate.com/download.php
You can trial PureSight free for 30 days and see if you like it. With PureSight you can set the page to be rejected based on it's probability of containing offensive material, you can set this on the server with a sliding scale.
Regards,
Matt