I suppose StumbleUpon would work as a recommendation engine.
http://www.reddit.com/r/nsfw/comments/7qz7t/reddit_please_he...
http://www.nakedtube.com/recommend
A little more about it:
NakedTube.com Revolutionizes Adult Video Discovery with Powerful Search and Recommendation Engines http://www.klixxx.com/new/index.php?option=com_content&v...
... lots more innovation to come :)
Ironically, your HN submission on this very topic was killed: http://news.ycombinator.com/submitted?id=redman
Maybe requiring a login to use the engine is too large a barrier of entry? We've been planning to offer an "anonymous" engine for non logged in users but don't have it slated until after our UI redesign.
It seems to me that is generally the case on the web. There was some discussion about login requirements as barriers here on HN recently.
You should allow use of the site without a login. Just have them log in if they want to save their preferences. Don't make email required. Why would I want to give my email address to a porn site?
Porn is often the first (commercial) industry to take advantage of many new technologies (DVD, the internets, etc), but even then there's probably tons of room to innovate.
For example, I recently saw a porn recommendation engine posted on Reddit that used collaborative filtering and all that fun stuff. AFAIK there aren't many sites out there that do recommendation, even though it seems pretty obvious.