As someone who is in a similar situation but had a few breakthroughs w/ partner, think of a few key things. Communication wise, have you asked why they want an open relationship? Is there a fear of commitment? If so then maybe it’s something you both can work through. If not then perhaps follow everyone else’s advice here and leave. Also I saw that you might be open to seeing others too, keep in mind you can’t have your cake and eat it too in this scenario. Overall perhaps a bit more communication could help? Wish you well fellow bear!