Most people do not like prenups, me included, so I can understand this being a sticky situation for anyone. You may want to make sure this is the person you want to be with if you think you need one.
I will tell you that there are ways you can protect yourself from having any of your money taken if you were to divorce, but it also depends on what state you are in. Some states are split it right down the middle when a couple divorces or you leave with what you brought into the marriage, or it can be a state where someone ends up getting more than the other person - a lot of times that ends up happening to the man and it usually is even more so when children are involved and most states give custody to the mother - otherwise it is the breadwinner that gets the biggest blow no matter if that is the man or the woman..

In my opinion, you should go to whomever holds your trust (not sure if it is a company, a private broker, etc...) and ask them about measures you can take to protect yourself.
I know that my parents have a clause in their will that protects me if I were to ever divorce. They also have it where I would get money initially and then the rest at a certain age - mine may be 40 as well. You would be safe from the money coming to you at 40 if you were to divorce before then.

I can completely understand your concern. Like another poster wrote, maybe ask your fiance if he were in your position what would he do? It's easy to say you wouldn't but maybe after some thought he would reconsider.
I would still go to whomever holds your trust and consult a lawyer as well about other options besides a prenup.
Hope I helped you in some small way. Good luck with everything!!