I have been there TWICE!
The first one I lost my virginity to and after it I said "Are we dating?" he said "No we are just friends that have sex." I got crazy in love with him but accepted we were just FWBs. Then events happened a year later and he said he didn't want to see me again, and I ended up meeting Mr Jaune.
While Mr Jaune and I were getting to know each other I still was in contact with the first guy. Mr Jaune didn't want to see me (I must be a great person haha) one week and so I went on a mini break and hooked up with the first guy. First guy started to hold my hand and kissed me all smooches. Well all I talked about was Mr Jaune....we stopped talking to each other.
Second guy well that is a long story, Mr Jaune was out of the picture I was a damsel in distress and I made a big mistake being with this guy. I did things with him I would never consider with Mr Jaune and by the end this guy did something to me which is left for a different story and this guy was falling in love with me and I was put in the spot of getting back together with Mr Jaune or being with this other guy. Mr Jaune was asking me out for coffee and he wanted to reconcile. Mr Jaune knew of the other guy but I didn't tell him much about the FWB relationship.
Any way with the second guy I see him a lot in town, we both ignore each other.
Over all FWBs/FBs in my experiance does not work someone falls in love and someone gets hurt.