Welcome to the forum. I think the best thing you can do is just talk to her and be honest. One of the reasons my husband and I opened to poly was my bisexuality. He doesn't want me with other men (nor do I want to be) but he understands that, both physically and emotionally, he cannot be to me what another woman can. And poly, rather than an open sexual relationship, is not only safer, but satisfies ALL my needs for a female partner, as well as providing him with a friend he knows cares about me just as much as he does and respects our relationship. I can't do the disconnected sex though so it was never an option. Be open, honest, and make sure you let her know this doesn't make her mean any less to you. Good luck.
I'd rather be hated for who I am than loved for who I am not.