From Network Sorcery (courtesy of Tao Security): RFC 3675 - .sex Considered Dangerous.
It's not what you think. It's actually a discussion of the reasons why we've not yet seen ".sex", ".xxx" or similar. Network Sorcery is the company which sells the RFC Sourcebook, a good-to-have for people who work with application and network protocols. They also have a pretty decent online reference for IP protocols, complete with header diagram and an explanation of the protocol.