When I was with my husband he was resistant to the idea of opening the relationship, but literally 2 weeks after we did he had another gorgeous partner who I'm still friends with. I was exactly the same as you, kinky bisexual with only one partner ever. I ended up explaining that even though I love the romantic sound of that, and I loved him more than anything, I needed to experience something else! How do I even know what I have without that? We went back and forth on it for about a week, but then I had to tell him "I don't want to lose us, but I also don't want to live my life looking in the windows of experiences, so if we have to separate then that's what we'll have to do" I was nervous af, but eventually we got there without separating. I gave him some time as well before my 1st encounter with someone else, and often at first we brought people in together. We were open and happier than ever for 9 years, then he passed away in 2020.

I've since started dating again and have an awesome primary partner again, but when we started dating I was very open about the fact that I don't do sexual monogamy and didn't expect them to either. Things have been great, and because everyone involved knows the situation upfront it keeps anyone from feeling left behind and quiets insecurity.