Honestly, I do not think you are in the wrong here. Protecting both of your assets by signing a pre-nup is beneficial to both of you. I can see how people confuse the suggestion of a pre-nup as a suggestion that the marriage may fail... but guess what, chances are likely that it will. I am not saying your relationship couldn't last a lifetime, but the statistics on marriage and divorce prove that those chances are high. It's my opinion that people who freak out at the idea of a pre-nup don't actually understand what a pre-nup is for. It's about protecting what is yours from before you enter a marriage, it has nothing to do with the strength of your relationship. I think signing a pre-nup is smart, and it's actually stupid not to sign one, for both parties involved.
To be honest, it could even appear that your girlfriend is selfish and using you for your money. I'm not saying that she is, but I'm saying there's more than one way to look at this, and you are not the bad guy here. Nobody needs to be the "bad guy" when discussing marriage and protecting your assets.
I think you should seek the guidance of a lawyer, so that they can explain how a pre-nup works. Suggest to your girlfriend that you want to look into it so that you can make a decision together, that suits both of you. If she isn't even willing to consider your position on this, then I think you need to seriously reevaluate whether this is a woman you want to marry. As much as it is important for you to listen to her and do your best to give her what she needs from you, she needs to do the same for you.
Hopefully you can try again to broach this subject to her. Try bringing it up at a neutral time, using calm language, and see if that gets you anywhere. However, do NOT just roll over and say, "Okay honey, we don't need to talk about this again", because this is a serious discussion you both need to have if you are serious about marriage.
"Caring is not an advantage."