https webfilter using a SecureNAT Client

Hi,

We are using ISA Server 2004 on Windows server 2003. Now we are facing issue while implementing https site filtering . We are able to block https://youtube.com and https://facebook.com using domain name set and URL Set but this is working only for clients who are browsing internet from proxy address ( 192.168.0.2 : 8080 ) or else using a Firewall Client...

If my client machine is browse the internet using a Secure NAT client , i mean keep gateway address as ISA Server ip , they are able to browse the https://youtube.com and https://facebook.com

So , my question is will SecureNAT Client not support web filter for https traffic ? Do i must have to use Web Proxy or Firewall Client ..

Please Help...

Thanks....