Well, according to the cleanweb.net FAQ, a site is blocked if it contains "pornography, depictions of sexual acts, violence, criminal acts, drug use, excretory acts, graphic medical images without medical context, discrimination, profanity, [or] adult humor." I'm guessing that iFeminists's Porn, Prostitution and Sexual Freedom directory probably links, at some point, to one or more of the above (at least by cleanweb's standards).

For example, there's sex-geek.com, which looks like a very interesting weblog written by a freelance writer and ex-prostitute. Wonder if cleanweb would approve of that?