I read in the webmaster section of a pornsite (Planet of Stars) that their convert ratio is 1:135   Does this mean for every 135 unique visitors to their website, one will join?  Or is it for every 135 clicks on their website in general, one will join?  Or something else?  What is considered the average convert ratio range for porn sites?

They try to make it seems it would be unique, but it's more likely clicks... or worse.

An average ofcourse depends how people are sent to the site (redirecting, banner clicks, after seeing a gallery etc.). TGP traffic is quite unproductive, something like 1:600 clicking from galleries.