It's much too early to declare the demise of marriage, but the Census Bureau tells us that it's no longer the top choice for American households.

By a very slim margin, the number of American families without a married couple at its head has outpaced the traditional family, according to a report by Agence France Presse. The number of nontraditional households has risen to 50.2%, while the number headed by married couples slipped below a majority to 49.8%.

The mix of untraditional families included households headed by single men or women, and 36.7 million households headed by unmarried couples.

Marriage, of course, comes with a range of financial perks, in addition to all the emotional ones. Insurance rates can drop, health care costs can be shared, and there's nothing more romantic than cozying up on the couch to read IRS publications about all the tax perks of getting hitched.

In fact, most of the country's laws assume families will be headed by married couples. That means unmarried couples need to think more carefully about how to arrange their joint financial lives.

That starts with the most basic question: How will the household expenses be paid? Living together requires the sharing of space and also of bills -- the rent or mortgage, electricity, groceries, telephone, cable, and the occasional late-night run for ice cream.

That means you'll have to decide how much each person will contribute to daily expenses and whether you'll each contribute equal amounts, or an amount proportionate to your income.

When my husband and I first moved in together, we were unmarried and had just bought a house. (He endearingly called me his co-mortgagee.) We planned to eventually marry, but we knew it would take a little while to get there. We agreed to each contribute the same, set amount each month to a joint account for household expenses and savings. That worked for us. Find out what works for you.

While you're at it, decide who will actually write the check, put the stamp on the envelope and put the bill in the mail. The best-laid plan will fall short if no one's responsible for the payment.

The questions get a little more complex from there.

Marriage can mean a new level of financial cooperation, even for couples who lived together before their wedded bliss. There are the Big Talks, about children or retirement or the plot of wilderness you've always wanted to own. Marriage also comes with financial traditions, of joint accounts and combined assets and shared budgeting.

That's not necessarily the case for unmarried couples, who may have to be more deliberate about laying out their financial expectations and goals. That's especially true when major decisions like home buying and retirement enter the picture.

You may think about setting out a written agreement, so both sides know the rules of the road. The level of financial comingling will probably reflect the commitment in the relationship and the financial quirks each brings to the table.

Like married couples, get to know each others' financial pasts and credit histories. Honesty is the best policy, and it's a timeworn cliche for a good reason. Better to tell your beloved about any problems in your financial past before they cause a problem now.

If you decide to buy a home together, you'll have to decide how to legally arrange the relationship. If one of you dies, will the other inherit the property, an option called "joint ownership rights of survivorship"? Or, will you be "tenants in common," meaning each of you own half the house and can will it to whomever you'd like?

You may even decide you need a durable power of attorney or a health care proxy, allowing your sweetie to make decisions on your behalf if you can't make them yourself.

Lastly, if you want your stuff, including that collection of rare bottle caps, to go to your partner in the event of your death, you'll definitely need a will. Married couples would be wise to have a will to ensure their spouse inherits their assets, but estate planning is essential for unmarried couples who want to leave their assets to their partners. Estate laws don't assume that unmarried couples want their partners to inherit anything.

Married or unmarried, Motley FoolGreenLight is the place to go for useful personal finance advice. Click here for a 30-day free trial.

Fool contributor Mary Dalrymple writes the checks and buys the stamps in her household. She welcomes your feedback. The Fool has a disclosure policy.