The simple answer is that it's a jurisdictional issue. Where you reside in marriage will determine the "ground rules" for any divorce proceeding. It's likely that any prenup such as you describe would be fairly easy to invalidate in California, for example.
It may also be that the place in which you are married may influence the outcome – this is much rarer, but especially for marriages that take place internationally, the validity of the marriage itself may be called into question, which could then affect the validity of the prenuptial agreement (if the nuptials are not recognized, the contract may be invalid, and common law could then be applied).
For what it's worth, I think you may benefit more from using a "Relationship LLC" model[*] for your 'marriage'. This would mean, effectively, that you would be "business partners" in your relationship, and subject to customized agreements that are made between the partners – rather than being subject to all the various aspects of existing marriage laws (which again, vary significantly based on where you live).
As an example, you could draw up an agreement that provided "shares" to each participant, that vested over time – if your fiancée wants half your assets, she would have to stick with the relationship for a specified period. This would also enable you to have a "buy out" clause – you could simply pay for her vested shares based on the length of the relationship and current value of mutual holdings. Alternately, you could even keep all your "incoming assets" separate, and only share those things that are truly shared – things like cars, houses, investments, etc that happen during or as a key component to the relationship. That way, even if you do a 50/50 split, it's only splitting those things that were actually shared between you, and not all prior and ongoing income/assets/etc.
It's not really simple (neither is regular marriage law), but it seems almost ideal for a situation in which you are pre-planning your divorce before being married.
And while I won't pass judgment, I will definitely advise extreme caution in proceeding either way. Even really "ironclad" contracts are dependent on the good faith of all involved parties to some degree… If you cannot trust your fiancée, then it may be that you will eventually lose everything, regardless of how much pre-planning you do.
Good luck!
[try here for a more detailed overview: relationshipllc.com]
deano.com 