Specific Dean/Sam Dom/Sub Fic - Found
Dean and Sam are in relationship, including a Dom/Sub one as well. They both get cursed, with Sam wing assaulted by people wanting to sleep with him and Dean being overly protective and taking things way over Sam's threshold as a sub.
Dean takes Sam to Bobby's where he keeps him drugged. Bobby realizing what is going on calls Rufus to help him find a way to break the spell, which they end up doing.
Afterwards, Bobby ends up letting it slip he knew about the boys relationship since he helped Dean setup an iron (ringed?) bed. This pisses Sam off because he told Dean he wanted the relationship kept secret.