Yes you can be sore for the next few days even if he just fingering you. That is completely normal especially if it is hard, rough or anything along those lines. On the FWB thing, honestly you really have no say there as far as who he dates, if he is fouling around etc... FWB is just that. A friend who you have casual sex with ever now and then, keeping emotion out of it. Years and years ago I had a great FWB we in fact where best of friends and we still are to this day. Once he got a steady gf though it stopped since in my opinion he would be fouling around on his gf. There was NEVER any emotion or jealous. Since he was just a FWB. Sounds to me like you have feelings for him whether you are willing to admit that or not. I am not talking about just friend feelings. I know you said he is not bf material, but sounds like subconciously you have a little more feeling then what meets the eye. I don't mean to sound harsh or mean this is just my opinion. Honestly you since this is just a FWB he cant be cheating on you, second I feel like I said already it sounds alot like you are gettting emotional feelings and becoming attached to him that is why you are feeling jealous. It is either time for you to remain just friends with no benefits or maybe rethink the whole bf thing. Becareful though, I have known a few people who had FWB and they ended up losing a friend in the end. That is of course completely up to the both of you. I wish you the best!!