On the February 20, 2005 episode of The Simpsons (titled "There's Something About Marrying") same-sex marriage was legalized in Springfield as a way to attract tourists. This results in Marge's sister, Patty Bouvier, coming out and announcing her intention to marry her partner Veronica, a pro golfer. When Homer learns how much he can earn performing same-sex ceremonies he becomes a minister and marries Patty (which is ironic since Patty, as well as her twin sister Selma, have nothing but contempt for Homer). Alas, it was later revealed that Veronica was a man.
Having same-sex marriage as a storyline on a mainstream TV show (a favorite with young men) was an important milestone in the fight for legalization of same-sex marriage. The attendant publicity brought awareness up a few more notches - anathema to the religious right, which frets whenever gay issues gain exposure.
This episode of The Simpsons aired a little more than a year after Massachusetts became the first U.S. state to legalize same-sex unions. And it just so happens that there is a city named Springfield in the state. However, the show's writers have never indicated in which state their Springfield is located (there are 38 cities, towns and townships named Springfield in the U.S.).
(For whatever reason Season 16 isn't yet available on DVD, just seasons 1 thru 13.)