I posted before. But I feel like I am missing something.

My partner (M37) and I (F36) have been together for almost 3 years(living together for 2). When we met I told him that I am open to anything in sex except for other people. He accepted my view, shared about his experience with FMFs and said “I don’t regret it, but they didn’t help those relationships”. Our relationship is a huge commitment for me and his huge happiness and relationships of a dream. He was obsessed over me and did everything for me to trust and commit to him. I love, respect and adore him. Sex is great. We both are honest that sex is best than with anyone else before. Few months ago he shared that he would like to have threesomes. I said I am way to possessive and jealous to FMF and have no interest in MFM. Now we are at a point when he says that his dream life is to live like Rikki Carman and JulieWild (who are the hosts of How to Have threesomes podcast). And he is not asking for it now, but says he may have to end out relationships to pursue that lifestyle. He says every other bit of our relationship is amazing. I am heartbroken. I have been imagining getting old together. He made so many promises to my kid recently about our future. I love him, but proposed lifestyle doesn’t excite me. I feel lied to. I am ready to give it a try and see if I like it. But I am heartbroken that he would give up our “magical relationship”. What do I do? Can a partner be more important than the lifestyle? If we have more than most people dream of why change initial terms? I am devaluing his fantasy?