I've dealt with this and other kinds of "referrer spam" quite a bit. Matt is 90% right. But I don't think the offending sites are trying so much to get webmasters to visit their sites themselves, since that's a pretty retail approach to garnering attention. Rather, they're trying to trigger server-side scripts that some sites (especially other porn sites) have that automatically create and display links to sites that send high referrals. That way search engines and your visitors will see the link, not just you. Either way, the stuff is really annoying (and depressing... just when you thought your site was becoming popular!)