The boys and the citizens of South Park are troubled by a large group of Harley-Davidson riders frequently making noise in town. Numerous complaints are made to the bikers, but they disregard this, seeing themselves as getting attention, and continue to make more noise around town. Frustrated, the boys and other schoolmates assemble to defecate on the seats of the motorcycles and they spray paint "Fags get out" on several buildings around town as well.
The school board, the mayor, and local homosexuals become outraged by the graffiti and take it as an homophobic message. The boys push to town hall to recognize the word, "fag", is used nowadays for a contemptible person ("An inconsiderate Douchebag" as Stan puts it) who rides a Harley-Davidson motorcycle. Much to the town's own surprise, they find the boy's argument makes a great deal of sense, and even the local gay community begins to put their support behind the changing of the word. An ordinance is passed in South Park, but is met with negative publicity from the rest of the nation, as well as heat from the bikers now being labeled as "fags" and drive off.
Facing pressure from the mayor, the boys decide to push to update the term with the help of the English Dictionary Officiates which is led by Emmanuel Lewis who is known for playing the character of "Webster" on television. An event is held in town, but the Harley-Davidson riders come together and riot to crash the event. After having inflicted a lot of damage to the town and scaring off the citizens, they corner the boys in an alley, but are then chased off and forced to surrender when South Park's gay men advocates turn their shotguns on them. In the end, the riders ultimately accept being called "fags".
Emmanuel Lewis, badly injured in the riot, declares the definition changed. The town rejoices and celebrates, and the show cuts to the updated definition of "fag", which reads: "Fag (făg) n. 1. An extremely annoying, inconsiderate person most commonly associated with Harley riders. 2. A person who owns or frequently rides a Harley."
Someone who is thinking about possibly buying a Harley is, of course, "bike-curious".
More on the substantive point can be found here.