Open Relationship

Hi ladies! I'm at a crossroads right now and I need some advice.. I have been with my fiancee for about 5+ years and he suggested a few weeks ago that we try an open sexual relationship. I've always felt like you should be fully committed to your partner, emotionally, physically, mentally, etc. But now hes convinced that people aren't made for just being with one person, our hormones arent made for that. Neither one of us has tried yet, but he really wants to and I'm just still in the mentality that its cheating. He strongly feels that sex is sex with other people and that he and I make love, our emotions are devoted to each other. Since hes proposed the open relationship hes been more lively, happy, enjoyable to live with but I'm not okay.. I don't like it I've never liked the thought of sharing my partner with others, I need advice and thoughts! We've been through a lot of unfortunate things. It's been two years since we've lost our first baby and I dont know if this is finally him dealing with it or him trying to find someone more capable of carrying a full term baby. Please help me here❤😏