Wow, sounds like you need to stay away from that person. It sounds like your dynamic (no matter what name you give it) is working for the 3 of you.
While I don't spend much time on fetlife, I have seen LOTS of poly folks on there. It is actually one of the few places poly is common. The BDSM community is quite large, if one small group is stressing you out, seek out another group. Block those that want to be nasty and negative.