The most popular tube sites paid a hard link on naughty.com (PR8). Check this "Naughty Friends" and you'll see everyone there! Atention on the terms "porn" "porn tube" "sex"... and compare it on google!
This naugty.com guy sell a link for thousands of dollars (anual fee). This goes against the Google guidelines!
Is possible report to Google on
http://www.google.com/webmasters/
If there are many complaints can work!
Sorry my bad english!