Couples disagree about money more than any other subject. The time to discuss your financial plans, philosophies, and economic life together is before you get married. The prenuptial agreement enables couples to develop a financial plan for the marriage; not just with the current financial picture in mind, but well into the future where finances are not yet known. Prenups are a way to confront money issues, arrange your financial affairs and solve problems before the marriage starts.
Consider this; do you know how your partner views money? Do you know their spending habits, how they view wealth, what their lifestyle needs are, and how they plan to achieve them? Do you know how they view themselves and how they view their own spending habits?
Still can't agree on whether to get a prenup? How about a compromise? Consider a "sunset clause". A sunset clause is a paragraph included in the prenup that lays out when the prenup will no longer be valid. For example, your prenup can say that the entire agreement becomes null and void after 10 years of marriage (or 15, 20, or 25). Burt Reynolds and Lonnie Anderson had a sunset clause in their prenup that rendered the agreement null and void if they had a child together. Sunset clauses allow the parties to express the expectation in their prenup of a long and happy marriage while at the same time guarding against a short-lived marriage. A sunset clause can serve to terminate the entire agreement upon the happening of an event or after a certain number of years of marriage; or it can terminate only parts of the agreement.
A sunset clause is also a wise thing to consider when the future is not certain. Ten or 20 years into the marriage, your entire financial picture may have changed and your prenup may be antiquated or out of touch with your current situation. The economic picture of your marriage is ever-changing and will certainly be different on the date of marriage as compared to 10 years later.