I see what you are saying in both instances but most end in the latter??? Curious.
i really dont know many FWB and have only done it twice, first I knew I would never fall for this guy in a million years but I needed it (SEX) and I wasnt ready to move on emotionally after a death. It lasted 6 months. the 2nd was very brief because he wanted more and I still wasn't ready and bailed!
But I dont know of any that have remained in a relationship or have started out as FWB.