Hooboy.
Ok Lotusart, allow me give you a similar situation but with a different outcome. Years before I met my wife, I dated and even got engaged with a woman (I'll call her "Cathy"). It didn't work out. But, we remained close. The breakup was amicable, we didn't tear each other down, and we stayed close. I moved away some months after we broke up (for unrelated reasons), but we stayed in touch. Years on, I met my wife. I told her right up front about Cathy, and that while there was nothing non-platonic between Cathy and I, and there hadn't been for years, Cathy and I remained very close.
My (not yet at that point..) wife took this in stride. She was ok with it. I think part of it was her general approach to such things, but a lot of it was that I was up front about it and never once gave her any reason to doubt my honesty about everything in our lives. My wife and I got married, and Cathy was at the wedding. Our kids refer to Cathy as Aunt Cathy. We're still exceptionally close. It's quite hard to describe really.
If my wife and ever been concerned about Cathy's presence in our lives, we would have talked it out. If it kept on being a problem, then Cathy would either be less of a factor in our lives or completely gone. I place my wife above all. I always have since we started dating, and always will. I would NOT be blowing up at my wife about this if it happened. She would have a right to feel as she did, and if I'm half the person I claim to be I would respect that. Yes, it would hurt to have to turn my back on Cathy, but I would do it.
To me, your fiance does not strike me as being up front and fully honest about this. If I were in your shoes, it would scare the heck out of me. Who, exactly, are you marrying? If he's not willing to calmly and rationally discuss this, I strongly agree with GoldCoCouple. This has alllll kinds of red flags popping up. It's less about this old girlfriend and more about his willingness to commit to you fully with absolute honesty. I don't see that right now.