Like the title says, I'm looking for "Nothing Else Matters" by poppymoon3.

Summary:
Sam and Dean were in their teens when John was struck by a steadily worsening curse. At first he simply needed regular sex. Then he needed to be fucked. Then he needed Dean or Sam to be the ones who fucked him. And with every passing day, John focused less on everything else and more on feeding off the sex, until the real world and their previous lives started to fade away, until even Dean and Sam's memories started to blur, the curse rubbing off on them as well.

Fast forward to the present day. Sam never went to college, choosing instead to stay on the hunt with Dean. There's only one thing that makes hunting difficult -- their nameless fucktoy. Sam and Dean have no idea where the mute pliant older man came from, they just know that thanks to his constant need to be fucked by one or both of them, none of them need to eat or piss or shit or sleep. They just need to fuck him, to let him blow one of them in the backseat while the other drives, for one of them to hunt while the other bends him over the back of the Impala.

Both Google cache and the Wayback Machine have nothing. On the Deleted Authors page here, the author didn't prohibit sharing their works, so I'm hoping someone made a copy of it.