Thinking I shouldnt be engaged. How can i forgive him?

So I have been engaged for a little over a year now. I am 21 years old. My Fiance is 25. We had dated a year and a half before getting engaged. Our relationship was wonderful, he was wonderful, we never fought, not even once! We were very respectful towards eachother. We were so in love. He was my first Anything ual. And i trusted him with that. Then the night he proposed he told me he wanted to marry me, he said i was the perfect girl for him, so laid back, understanding, sweet, everything he could ask for in a girl. He said we were destined to be together, it was fate, i was his soul mate. I absolutely melted! Well after all that he started telling me how I was also perfect for him because i would allow him to be with other women and have other relationships, and have with other women, because i was open to the idea while we were dating ( fyi thats what i believe dating is about, i told him as long as he was upfront and honest immediately if he met or slept or did anything with another girl, and he knew that when/if he told me i would have to decide how i felt about it, whether i would stay with him or not..no hard feelings because again its dating,so basically i would not have considered it cheating at the time because that was our agreement) I was so caught off guard and caught up in the wonderful feelings that i felt when he said he wanted to marry me, that i just didnt say anything. I was shocked into silence, my heart sank, i felt sick. But he made it sound so convincing and so easy that i ended up accepting his proposal, after all I do looooove the man. However, I was left felling sick still about the rest. He wanted to be polyamorous. Not at all what I wanted,fine for dating, not for marriage or engagement. I expressed this to him and he made me feel guilty, and like i was being selfish for wanting him for myself, and like i was being judgemental, and closed minded. I will admit i was caught up in our love, and tried to accept things that i never should have given a second thought to. We continued discussing/arguing about this for days while we considered ourselves engaged already. The same night he propsed he BEGGED me to get a job so that he could show me off to all his army buddies, and so i would be more attractive and as he put it"HOT." That completely crushed my heart, and put a large dent in my self esteem, fyi i have never been self-concious up until that moment.Never would i have expected this from him! I said absolutely not. He brought it up a few more times other occasions then finally dropped