One of the most pointedly satirical episodes of South Park, dealing with the hot issue of sex education in schools.
One of the main themes of South Park is that the parents are all very willing to launch a moral crusade but refuse to be taken to task for it, and don't want to do anything about it (as in South Park: Bigger, Longer and Uncut, in which the parents overreact and just use censorship of Terrence and Philip to avoid blame for the kids' profanity). This is embodied in this episode: they wish to shift all responsibility for their childrens' sex education onto the school, as they are either too lazy or too embarrassed to do it themselves. Chef plays the role of the voice of sanity, who argues that the parents should do it themselves rather than being idiots about it.
Oh yeah, spoilers are coming up...
In the episode, Cartman teaches the other boys how to beat off a dog while saying "red rocket", a technique he learned from 5th graders. The parents see this and, unwilling to explain the facts of life, cry out to the local school to teach for them. The classes are split into two: the boys taught by Mr Mackey, who hasn't had sex for 21 years and can't remember where he put his dick when he did, and the girls by Miss Choksondik, who has insane right wing views on sex of any kind and force feeds the unprepared students the view that they can get AIDS if a boy doesn't wear a condom, specifically leaving out the bit which says you have to have sex first. The boys, who the girls are now scared of, are forced to wear condoms all the time, and after twigging that the girls can give them diseases as well end up resorting to drastic measures (i.e firebombing the girls).
Of course, the moral message of the show, as summed up by Chef, is that it is utter folly to sign over control of your childrens' sexual education as you don't know who is teaching what: your kids could be taught by "someone who doesn't know about it, someone who has a bad opinion of it, or a pervert". Like most of the satirical South Park episodes, the show manages to destroy its opposition's arguments with pinpoint accuracy, saying that the only reason parents support sex education in public schools is because they don't want to do it themselves, and that it should not be taught with "textbooks and diagrams". It's an excellent, and hilarious, episode, if only for the fairly worrying sight of Cartman jacking off a dog and Chef's steadfast belief that the age for anyone-anyone-to become sexually active is 17: "Even if you're not ready?" "Yup! 17". It was, in fact, so controversial, it was banned by Sky Television in the UK on its first run, and has not been shown again since on that channel (Channel Four continues to show it).
"Beating off the dog is not appropriate when we have company... uh I mean ever. Beating off the dog is not appropriate ever."
TV Tome has an excellent page on the episode at http://www.tvtome.com/tvtome/servlet/GuidePageServlet/showid-344/epid-63618/