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.
I tried to use it, got annoyed at the signup form, filled it in with a throwaway email account, then gave up when it expected me to go click a link in the email account before i could use the site.
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?
Ironically, your HN submission on this very topic was killed: http://news.ycombinator.com/submitted?id=redman