It's not every day I get to write about gay cartoon rats here on Collider. It's also a rarity to talk about Arthur, the kid's animated educational series that's been airing on public broadcasting channels around the world since 1996. And never before have those topics crossed over, but today is that day. On the long-running show's recent Season 22 premiere, an episode watched by millions of viewers in the U.S. alone, a character who has been with the show since its very beginning came out as gay and was wed to his …