(LifeWire) -- First comes love, then comes marriage -- or at least that's how the saying used to go. An increasing number of heterosɛҳuąƖ couples have been shacking up without plans for a trip down the aisle.
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Angelina Jolie and Brad Pitt are raising four children together. Pitt has said the couple won't consider marriage until same-sex couples have the right to wed.

According to a 2003 U.S. Census Bureau report on families and living arrangements, 4.6 million U.S. households were occupied by unmarried couples of the opposite sex. That's up from 2.9 million in 1996.

"People don't question, 'Why did you get married?' They only ask you, 'Why haven't you gotten married?' " points out Marion Willetts, an associate professor of sociology at Illinois State University. "I think a lot of people feel if you were really serious about your partner and your relationship, then you'd get married."

But from what Willetts has found with the life partners she's spoken to, that is simply not true. "They're in this for the long haul," she says. "This isn't just some convenient thing or trial marriage. They're just as committed to their relationship as married people."

While Census data doesn't identify couples who have sworn off marriage, Willetts has followed this group in a study of 83 couples she's surveyed over the past few years for her current research, tentatively titled "Union Quality Comparisons between HeterosɛҳuąƖ Licensed Domestic Partners and the Legally Married." According to her research, it's not just 20-something social idealists who decline to get married. Willetts has interviewed octogenarians in committed relationships who are not planning to tie the knot.
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In fact, according to a 2003 AARP survey of 3,500 single Americans aged 40-69, just eight percent cited finding someone to marry as their reason for dating, while 49 percent said they were looking for someone to talk with or do things with -- in other words, a companion.

As the prevalence of unmarried couples rises, more companies and state and local governments are providing a range of domestic partner benefits. Some, like New York state, require that medical benefits be extended to both same-sex and opposite-sex domestic partners, according to the Human Rights Campaign Foundation. Other states, like Illinois extend such benefits only to same-sex partners.

Why do some couples opt for long-term companionship instead of legal marriage, like movie actors Goldie Hawn and Kurt Russell, who have been together since 1982?

Alison Hatch, 30, a part-time instructor and doctoral candidate in sociology at the University of Colorado at Boulder, and Daniel Welch, 31, an elementary school teacher, cite reasons both social and political.

"We are philosophically opposed to marriage for a few reasons, but mostly because we do not feel as though we can in good conscience enter into an institution that actively discriminates against gαy and lesbian couples," says Hatch, who has lived with Welch for four years. "It is our belief that it is inherently discriminatory to grant social benefits to some couples and exclude others."

Actor Brad Pitt told Esquire magazine in October 2006 that he and actress Angelina Jolie -- with whom he is raising four children -- have declined to consider marriage until same-sex couples have the right to wed.

Partners choose not to marry for other reasons. Hatch, whose dissertation examined 48 committed heterosɛҳuąƖ couples choosing to not pursue legal marriage, has spoken to people who feel marriage is a patriarchal institution. Some are against the intertwining of church and state in marriage. There also are emotional reasons why committed couples choose to stay unwed.

"I think that a lot of people like the kind of organic nature that your relationship takes on when you decide not to marry," says Julie Bluhm, 31, a Minneapolis, Minnesota-based clinical social worker and a board member of the Alternatives to Marriage Project, a national nonprofit organization advocating equality for unmarried people. "It's almost a deeper appreciation of their relationship and the privacy of it."

Being unmarried was not necessarily an obstacle to parenting for a small group of couples in Willetts' research. In a study published in the Journal of Marriage and Family in November 2003, two of 23 couples Willetts surveyed had children biologically related to both partners, though several couples had children biologically related to one partner -- typically the result of a previous marriage.

Of the two couples, one pair had been living together for over 15 years and did not encounter any legal issues concerning their children by not being married. Said Willetts, "They expressed to me that everyone -- including their children's friends -- had just assumed they were legally married anyway."

It's disgusting I tell you. Another blow to the institution of marriage which God Himself established.