You keep mentioning not pushing him too much or using language that includes certain words such as poly. What's wrong with using a language that works around this stuff? That is why it has been created and formed.... so people can use it to express themselves.
If you love this woman and love him, then why are you not willing to face up to the hard stuff and start creating a better situation for all of you? You seem resistant to any suggestions. Your commitment to both of them seems kind of half ass to me. Like you want to brush it off and hope it goes away without you dealing with it. Even your coming here to write about it and seek advice seems half ass. Do you want our suggestions or are you here to prove everyone wrong and that you'll be just fine? It sounds like you aren't fine or you wouldn't be here.
Have you read any threads on jealousy? I am finding that you want a quick answer for something very complicated and involved. This is not something that you can just plaster up and forget about. Jealousy runs deep. Its not a matter or saying "just get over it."
If it doesn't work for him to read stuff because of whatever reason then find out for yourself and share what you find. You seem to just want to dismiss it all and not talk about it. I highly doubt that you will get to the bottom of it if you aren't willing to talk about it in some way that moves him forward into solving some of his issues. There is a reason he is jealous. Its not coming out of thin air. Jealousy is made up of many emotions that stem from years of experiences and traumas and you seem to just want to brush that aside.
As to the wedding. Why marry if you aren't into it? You are getting married for your parents? What?! Are you serious? Why? You seriously think this is going to work if it is for someone other than yourself?
I sense a large portion of denial in your posts. It makes me wonder how much you actually deal with some of the hard stuff in life or respect that others go through stuff. Does your fiance feel the same way about this wedding? I'm completely baffled as to what you are here for at this point.