Advisors will inevitably have clients whose marriages end in divorce—the numbers practically guarantee it, according to attorney Daniel S. Rubin.

That's why preparing clients for the possibility of a divorce, with tools such as prenuptial agreements, is so important, Rubin, of the New York law firm Moses & Singer, told an audience at the FPA of Metro New York's 20th Annual Forum in Manhattan.

The case of one celebrity, he said, perfectly illustrates the point: Paul McCartney.

"He thought that all he needed was love, but what he really needed was a prenuptial agreement or some other form of premarital planning," Rubin said.

Census data shows there were some 1.2 million divorces in 2009, and that between 40 percent and 50 percent of American marriages end in divorce, he said. About 10.2 percent of the adult U.S. population comprises divorced people; about 40 percent of first marriages, 60 percent second marriages and 73 percent of third marriages end in divorce, he added.

Rubin, in a session entitled "Protecting Your Assets from Your Future Spouse," argued that advisors should counsel clients who are about to wed to consider prenuptial agreements.

Citing the case of McCartney, Rubin explained that the former Beatle failed to strike such an agreement with his ex-wife Heather Mills and ended up paying $48.6 million in a divorce settlement after four years of marriage.

The probability of a first marriage ending in separation or divorce within five years is 20 percent and after 10 years about 33 percent, according to the National Center for Health Statistics.

Rubin said prenuptial agreements are needed because divorce laws can be "murky." Divorce court judges, Rubin added, "have a tremendous amount of discretion in the highly subjective area of property division in the event of divorce."

An example of this, Rubin explained, is the New York State Domestic Relations Law, which gives courts a lot of leeway in disposing of property. It states that in "determining an equitable disposition of property ... the court shall consider ... any other factor which the court shall expressly find to be just and proper."

The law can be "particularly vague" in "critical areas involving subjective valuations, such as with business interests," he said.

Prenuptial agreements are critical to avoiding these problems, he said. They can "enable couples to decide for themselves any number of issues and to write down their own contract to account for, amongst other things, the financial consequences of a divorce," he said.

He added that prenups make sense because they are normally written at a time when "the parties are likely the most agreeable towards each other."

Prenups should be done as early as possible, Rubin said. The closer people are to marriage, the more difficult it is to create an agreement, he said. Once couples are married, he said, prenuptial agreements are usually unachievable.

But prenups are widely recognized and can avoid most of the economic problems of a breakup.

"Properly executed prenuptial agreements are valid and enforceable in all fifty states and the District of Columbia," he said.

He noted that New York is considered to be particularly hospitable to prenuptial agreements. One way to help ensure these agreements are legal is for both parties to each have an attorney, he said.

"Independent counsel is not strictly required, but it is the surest way to show that a prenuptial agreement is entered into knowingly and voluntarily and without fraud, undue influence, duress or overreaching," Rubin said.

Rubin said some people may object to signing or even discussing a prenup, with some viewing such an agreement as "unromantic."

Nevertheless, he argued that the inability of a couple to even talk about a prenup agreement probably provides more of a reason to have one.

"If one feels that one cannot discuss difficult topics with one's future spouse," Rubin said, "it would not seem to bode well for a successful marriage.