I totally get the pheromone thing! So much so that I've only had lasting relationships with people whose skin (not soap or perfume or cologne) smells good to me. It's very weird to me that it's *such* a strong attraction for me.
I sympathize with your struggles with your fiance. For a while, I was dating this guy I really adored who was monogamous, and wanted to be monogamous with me. He really couldn't wrap his head around the idea that I'm happy with more than one person in my life, and kept trying to convince me that my open/poly relationship with my boyfriend was abusive to me (?!), that I should let him sweep me away for a nice little conventional marriage.
I finally had to cut off contact with him, because my capacity and desire for multiple relationships was so very hurtful for him, and he couldn't seem to find peace or let me be who I am. I also knew that I couldn't let him continue to hold onto hope that he could "convert" me.
I agree with MonoVCPHG - it's important to figure this out now, before you get married, and it's good that you're trying to find a way to be authentic before you make promises and commitments to each other. I think if you feel like being monogamous would be being "stuck," then you may want to take some time to decide if that's a route you can travel without resentment, bitterness, despair, or restlessness down the road. Ditto for the fiance, but the other way round
I'm not sure where you're from, but something that really helped me understand polyamory in a tangible way was to attend some local poly community events. It's one thing to read about it, but seeing someone and their girlfriend and their girlfriend's boyfriend and their three kids eating couscous at a potluck makes the whole thing seem much more normal and less bizarre and less sexually charged. That may be way too much for your fiance right now, but it's something that was beneficial to me.
I wish you luck that you and your fiance can come to a point where both of you are happy and fulfilled
And welcome to the community from one newbie to another!