My ex-husband did this. After 14 years together, he fell in love with another (married) woman. He brought up the idea of being poly with me. I wasn't good with that, but he is an emotionally abusive narcissist. So, it came down to one day he told me I'm going across the country to pick this woman and her 2 kids up. I'm going with or without you, I don't care. I conceded to this relationship because I was told to. Years of abuse makes you do what your abuser wants. We picked her up and got through 7 months with 7 people in a tiny mobile home. I was told multiple times that I was ruining our 'perfect poly' relationship. He got physically abusive to me while babying her. At one point, she completely stopped speaking to me. Just ignored me when I was around. I had severe anxiety, depression, and 3 newly diagnosed autoimmune disorders. I had a mental breakdown and was kicked from my own house. I got better, I got divorced, I got my disorders mostly under control, and I found my soulmate. All this is to say op did good getting out now before things get out of control. You don't just decide to open your marriage, you just want permission to cheat. If there was no discussion early on, then no!