V LONG - SORRY:
I read this yesterday, and have been thinking about it since. I haven't reread so can't name them, but some of what the previous people have said make so much sense. My dh asked me to sign a prenup before getting married, (though he's nowhere near a millionaire, he had far more than me), and I had the same kneejerk reaction that your fiance is having.
It killed the romance of the engagement for a while. At first I said no problem, but then as he kept reminding me to see a lawyer (as someone said she needs to get her own legal advice) I was feeling more and more pressured, also wondering what happens in 10 years if he meets a younger model etc etc.
Anyway, I then looked at our relative situations; he owned his central london flat outright, and I was single mum of 2 little kids, and at uni. He already was taking on my kids financially as their father has never paid a bean, and me while at uni, although I was bringing in a bit with part time work.
I went and saw a lawyer (which he paid for) and the lawyer advised me not to sign one, but also had a look at our draft agreement and suggested wording changes because the initial one said I could never claim anything. I came back from the lawyer and told my dh the lawyer thought I'd be mad to sign it as we were thinking of emigrating and my kids and I would have no protection at all.
After discussion (and tears) we negotiated and it came out that the main thing that he wanted ringfenced and protected was his flat, and his pension for the years before we met, and he realised that I felt insecure because of my lack of savings, and that I would feel like the second class partner.
Once he acknowledged how I felt, and assured me that while we are together everything would be shared and I would have as much right over earned money as he did, and that we would set up a savings plan to make me feel more secure, I realised that that was pretty well fair enough and I felt much better about it. I signed one that we worded together. Anything built up while we;re married gets shared.
I've hardly even thought about it since then until reading this over 2 years later (and we've emigrated). If we ever get divorced I don't think I will regret this, as I don't want more kids and want to find a good, rewarding job. I will get a share of his pension for the years we are together. We have got a savings plan which builds up every month, and I'll get half of that.
He is not stingy with money at all, and I can spend from the joint account where his salary goes without restriction and I do, not just on household stuff, but on fun, discretionary stuff. He sold the flat to buy a house where we now live, and the remainder (I think around 3 or 400K is in a savings account which I vaguely know of but don't know the details of. He did tell me, and all his documents are available in his study for me to look at if I want, (we don't have a safe) but I don't and I've already forgotten because I didn't take much notice as I don't think of that money as mine.
His will leaves everything to me or the kids of I die first, and I know if I die first my kids will be taken care of.
Anyway, this has been a bit rambling and I've been remembering bits as I go along as the details were at first fuzzy, but what I am thinking is that you are not wrong to want to protect what you have earned prior to meeting her. I think you just have to talk very honestly, and also it is very important that she knows that she will be an equal partner in the marriage and can make financial decisions that affect both of you (just not with your prev. cash). It sounds like you're already doing this. Maybe you can show her this post, as I know how she feels and came round to feeling that it was fair.
At the end of the day, I love my dh and I trust he loves me too. But so did the majority of people who got divorced, yet people are always surprised at settlements awarded to ex partners who contributed substantially less (obviously, childcare has a financial value). It happens, and I'm happy that I won't get any MORE than I'm entitled to, as all I would want is a fair settlement. Half of what we build up together is fair. As this is our first year abroad, we are spending every penny of his salary, but when I am able to work (in a few months when permit comes thru) this should improve.
I've tried imagining if he has an affair or he requests a divorce for no good reason, and firstly I can't really see it happening, as I trust in our relationship, but secondly, even if he did, I can't see my subsequent rage being satisfied by getting his flat money, or his pension, and having him hate me for that. I've always been the type of person to just not demand anything once relations have broken down (ex friends / partners etc) as I don't want them to think I need them in any way. I have proved I'm not a gold digger and that I trust him, and he has so far proved to me that he's not possessive about money and trusts me.
Anyway, good luck and I hope it all works out for both of you.