Specific fic: Dean/Castiel, Castiel is an incubus (FOUND)
When Dean and Sam figure out what Castiel is, they lure him into their motel room, planning to kill him. Gabriel shows up and freaks out, Castiel goes crazy when he thinks Gabriel is trying to take Dean away from him, and eventually they decide not to kill either of them. At the end of it Dean and Castiel wind up having sex in the motel bathroom, while Sam takes Gabriel to sleep off the sleeping pills in the Impala, and the brothers both agree to let Castiel and Gabriel come along with them.
If anyone can help me find the fic, I'd be super grateful. If I recall correctly, there were some mild undertones of Sabriel - when Cas freaks out at Gabriel, he winds up wrapping himself around Sam's legs and I think licks his cheek so that Cas will associate his scent with someone else.