Forget that old adage, first comes love, then comes marriage. These days, it's more like, first comes college, a good job, maybe a house and a savings account—and then we can talk about marriage.
As a battle rages over the rights of gay and lesbian couples to get married, experts say the share of heterosexual Americans who are married has fallen dramatically compared to decades past. What's more, the demographics of who is walking down the aisle also have shifted substantially.
In recent years, people with a college degree have become more likely to get—and stay—married than their less educated counterparts, and those who stay married also tend to be much wealthier than unmarried adults.
"Some people have talked about marriage as a luxury good," said Susan Brown, a sociology professor at Bowling Green State University and co-director of the National Center for Family and Marriage Research.
That's a stark switch from decades past, especially for women, according to the research center's analysis of government data on women's marriage patterns by education.
Back in the 1940s, college-educated women were the least likely to be married. The opposite is true now. As of 2011, around 60 percent of women with a college degrees were married, compared with less than 50 percent of those with a high school degree or below, the analysis found.
(Read more: Work may be tiring, but it beats parenthood)
"What we're seeing with marriage trends today mirrors what's happening in our broader economy, where we're seeing diverging fortunes for those at the upper and lower end of the spectrum, with rising inequality," Brown said.
Some experts argue that marriage itself is contributing to rising inequality, because people who are highly educated—and therefore have higher income potential—are more likely to choose each other as spouses.
That's making it less likely that marriage itself will move someone up the economic ladder, and increasing the chances that two low- or high-income people will couple up and share their economic struggles, or fortunes.
"The doctor used to marry the nurse. Today, the doctor marries the doctor," said Bradford Wilcox, director of the National Marriage Project at the University of Virginia, which has documented the rapid decline of marriage among people with just a high school degree.
There are lots of theories for why people are increasingly choosing spouses with similar educational backgrounds—or feeling like they aren't in a financial position to choose marriage at all.
(Read more: 'Treading water': Saving for retirement is harder)
"We're setting a higher bar for marriage today," Wilcox said. "That bar is not easily met for working class and poor women and men."
Christine Schwartz, an associate professor at the University of Wisconsin in Madison who studies the relationship between marriage and inequality, said one factor seems to be a growing preference toward egalitarian marriage. That's in contrast to the old model of dividing tasks—the husband goes to work while the wife takes care of the domestic tasks.
Another factor may be that both men and women, subconsciously or not, are looking for a spouse who can provide financially.
"There's the economic reality that people ... often feel like they need two earners in the family to meet a given standard of living," she said.
(Read more: From low to no wage: Shutdown hammers working poor)
Education and social status have always been attractive qualities in potential husbands, said Jeounghee Kim, an associate professor at Rutgers University who studies marriage trends. But starting around 1990, she said, "(Women's) economic status began to work pretty much like men's economic status: The more education you have, the better economic prospects or careers you have, you are more likely to get and stay married."
Her research has shown that higher levels of education, and thus income potential, also greatly reduce the likelihood of divorce—although that effect is stronger for white women than for African-American women.
It's not just that college-educated people tend to choose each other; experts say they also are more likely to choose marriage at all.
Brown said that's partly to do with a growing sense that just to be marriage material, a person needs to have already met certain financial milestones, such as going to college or having a good, stable job. For many people with low education and economic prospects, she said, those can become insurmountable hurdles.
"It's not for lack of wanting to get married, or lack of interest," she said. "I think it's a feeling that they're not in a stable … point in their lives."
Instead, many people are choosing to live together without getting married, and increasingly to have children without being married. But experts say that in the United States, living together is is not proving to be a replacement for marriage.
"Cohabitation has not become marriage-like," Kim said. "They're much less stable. They don't last nearly as long."