I think I read a story a while ago where some sort of beastie curses Sam & Dean with a fuck or die. Standard, but the twist was that they were already in a relationship, and nonchalantly got down to the sex.

I think it was on AO3, but I could be wrong.

Thanks!

Five Times Dean and Sam had to F*ck-Or-Die by smallcaps.

Thanks adamantina_lj!