First, you CANNOT download porn videos and save them on your iPod Touch, but you CAN stream/watch them over Wi-Fi. See Free iPod Touch Videos.
The biggest issue with free iPod Touch porn downloads to your PC/Mac, is Viruses and Trojans! My advise is to steer clear of any site offering free iPod Touch porn videos. It's simply just not worth the hassle.
If you're after iPod Touch Porn Downloads and you have $1.00 to spare. Get to a PC/Mac and try the 'one day trial' offered by the Brazzers Network (Note: You need to click this link on your PC/Mac, as it will detect that you're using your iPod Touch and send you to the Mobile Version of the site. A pain in the ass!
These guys operate the Brazzers Mobile QuickTime porn streaming service for iPod Touch and iPhone users. The one day trial gives you 24 hours access to their whole library of iPod Touch optimized porn video downloads.
Sign up, and download all you want to your PC/Mac. Then sync it to your iPod Touch using iTunes.
xratedtouch.com provides links to porn for viewing on the iPod Touch. This site is in no way endorsed by, or affiliated with Apple - Site is best viewed using an iPod Touch via a wi-fi connection.
