性交 (xingjiao) is the act of sexual intercourse. The term is used in sex-related words, including 近親性交 (incest / jinqinxingjiao), 性交体位 (sexual positions / xingjiao tiwei), 三人性交 (threesome / sanrenxingjiao), and others, all of which are banned.
Why it is blocked: Looks like some things haven’t changed since 2005 (when xingjiao was discovered to be blocked on instant messaging programs). A cursory test shows that most of those words are still blocked, though some critics contend that doesn’t stop young folks from using the service for illicit purposes.
