I feel for you. I myself am in an open relationship. Although you could consider me the side of their relationship. Anyway, he was happy for me to have sex with others and play the field initially. Then a few months ago wanted to keep me for himself. I haven't dated anyone else because for me I just have to have that intellectual bond as well and don't have sex for the sake of it. Perhaps she is catching deep feelings. In my opinion, over time I think when you settle into a routine, someone either becomes more deeply bonded and others become bored. The dynamic is hard. I used get jealous when he talked about his partner but then i knew what I was signing up for. We can play naiivity here but everyone eventually catches feelings and wants to just become two. There are the odd exceptions. I'm sorry you have been feeling this way it doesn't feel nice. I would think either explain your feelings clearly that you no longer want an open relationship (which maybe you're scared to admit?) or at least change the rules.
