If you have an unused domain and you're willing to put a bit more work into a new use for it, then you might want to consider setting up a "porn directory/review site", as offered by the BrainCash affiliate program.  After signing up as an affiliate of BrainCash, you'll have the option of going to their "Review" section and setting up your own adult site directory.  The pages are hosted by BrainCash and they put their own codes on a percentage of the links within the directory. 

Making this profitable will require some work on your part, however.  You'll have to sign up for all of the affiliate programs for the sites that appear in the directory and then enter your affiliate codes into your BrainCash account.  It's not hard, just time-consuming: there are hundreds of sites that will be listed in your directory and more will be added every week.

We've used this option on one of our own sites, so feel free to contact us if you have any questions about setting it up.