You have a reason. You are not attracted to her. Your husband needs to suck it up and let go of this kumbaya, rainbow coloured pixie dust fantasy. If I were you, I would sit her down and tell her that while you respect her as a person and whatever, you want to end the intimate side of things and just be friends and cordial. She can transition from your girlfriend/romantic interest to your metamour and call it a day. Her feelings may be hurt, but she will bounce back. You just wish not to be involved in the sexual aspect, and that is your right. It is your body. Your husband does not have the right to get mad about what you want to do with your body.
You said they dated while you and your husband were apart. Did they ever stop dating, or did it just continue while you and your husband worked out your issues? The foundation and introduction to poly make all the difference in the world.
Just know that your feelings are valid, and you have the right to say no.