I think the decision to leave or go is entirely up to you. If you are resenting feeling "sexiled" quite a bit, tell him that and recommend he and his FWB go get a hotel room.
However, if he plans to keep seeing this person, hotels can get expensive.
My gf and I moved in together 6 months ago, and we made sure to find a big enough house so that we each have plenty of personal space. We are in a ranch house with a refinished basement family room, and a guest room. She has only had one other person over, a play partner, once. They used the downstairs family room.
My bf, who is also her FWB, comes over 2-3 times a week and she has no problems with him and me having sex in the living room, our master bedroom, the family room, or the guest room. Or the bathroom, the kitchen... Once in a while we all have sex together.
Do you also have FWBs? You only see them at their houses?
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37