Actually, the slash fiction I saw had Casey hooking up with Donatello, during that sequence where they were fixing a truck in the first TMNT flick.
Not that I purposefully went looking for TMNT slash fiction. I swear it was just one of those things someone mentioned in passing, and then I had to go and see the horror for myself.
Wasn't a terribly written thing, honestly.
 
    

When evil spreads across the land, and darkness rises and the monsters roam. When the creatures of the night make beautiful music, and the things that go bump in the night go bump with greater enthusiasm. When the world is in peril and is in need of a hero...

These guys are, sadly, the best the world can hope for. These are the adventures of the heroes of CVRPG. They mean well, they try hard, and occasionally they do the impossible...

They actually do something heroic.