When Angelina Jolie and Brad Pitt went to consult a divorce lawyer, they weren't getting a divorce, per se. Rather, they were going to sign a dating prenup -- the newest way for committed-but-cautious couples in the flush of first love to protect their ass...ets.

As the Telegraph reported in January, "The contract was like a tailor-made version of a pre-nuptial agreement except for an unmarried couple's split." While most unmarried cohabiting couples don't have six kids, fortunes of $205 million, or multiple homes, Brangelina arrangements are not as uncommon as you might think.

The unmarried committed relationship is on the rise. In 2009, there were 6.7 million unmarried opposite sex couples living together in the U.S, spawning a generation of are-we-or-aren't-wes. I've written about them in length in my book: A Little Bit Married: How To Know When It's Time To Walk Down The Aisle Or Out The Door. And, out of the burgeoning cohabitation movement comes another potential relationship pitfall: The dating prenup.

This new relationship ritual can run the gamut from legally binding to purely informal, and govern how committed-but-unmarried couples in their twenties and thirties are deciding who gets custody of the dog, the Xbox, friends, and furniture in the event of a break-up, reported The New York Post recently. But are they a good idea?

Bonnie Weil, the author of "Make Up, Don't Break Up: Finding and Keeping Love for Singles and Couples", didn't want a dating prenup when she moved in with her now-husband. "We could have easily drafted one up because we each came in with existing property," she says. "But we trust each other, so we didn't feel it was necessary."

Weil, who is also a family therapist in New York, says the surge in dating prenups is reflective of the mentality today that divorce is always an option.

"I see a lot people in their thirties say, 'I want to have a pre-nup in case it doesn't work out,'" she says. "It's a problem if people go into a relationship thinking it might not work in the first place."

Maura Kelly, a writer in her thirties who blogs about dating at A Year of Living Flirtatiously, says she wouldn't move in without a dating prenup. "I think they are a great idea," says Kelly. "It's easier to negotiate rent -- and whether you or he pays for the free-trade coffee -- before you decide to move in together."

So how far should a prenup go? Should they cover every nitty-gritty aspect, down to who pays the parking tickets? Kelly thinks so. "Why not go the whole nine yards and itemize everything?"

But, wait, before you grab the indelible ink...

While many a couple thinks they are being prudent and mature by drafting a dating prenup, Shannon Fox, a family and relationship therapist, sees a darker side.

"People who are considering a pre-nup before moving in together, are not ready to be moving in together," she says.

To that point, Fox was working with a 26-year-old divorced woman who wanted to create a dating prenup before moving in with her new boyfriend. Her reasoning was: "I just can't put myself in a position to get so screwed again."

"What she didn't realize," says Fox, "is that no piece of paper could earn back her trust." (And Fox refused to draw one up for her.)

That, in a nutshell, is the catch-22 of the dating prenup: If you're a couple convinced you need one, are you forgetting to itemize the elephant in the room?

Fox says the decision to draft one raises some bigger questions: "Is the fervent desire to protect themselves indicative of unresolved issues within the relationship? Does this person lack faith in his or her ability to choose a trustworthy partner and instead want to rely on a legal document to protect themselves from another bad relationship decision?"

If you are Brangelina, the answer may be yes.

Hannah Seligson is the author of A Little Bit Married: How To Know When It's Time To Walk Down The Aisle or Out The Door.