The rising 'one' population

Yu said she has two very good friends who are single. "We hang out together every weekend, and I enjoy being with my friends more than going on a date with someone I just met — that's why I think I will be not actively looking for a relationship in the next few years," she added.

Coontz said one reason people are waiting longer to get married is because more of them, especially women, are going to college. Even after graduation, most people want to wait until they have paid off at least a good portion of their college debt and acquired a steady job, she said.

Two earners are increasingly necessary for people to establish a middle-class lifestyle, so most individuals want to make sure that they and their partner have decent earnings prospects.

"Another part of the answer is that our expectations of marriage — especially women's expectations — are much higher than in the past, and their options outside marriage are greater," said Coontz. "So, they can be more 'picky' about their mates, and they can afford to wait until they find one who suits them."

She said women, and men too, want someone who can earn a decent salary, but many women now rank other factors higher, such as willingness to share housework and child care.

Coontz said that the increasing inequality in society means that many low-income individuals — male or female — aren't seen as good marriage prospects.

"Often, couples who are for each other will cohabit in order to save money or explore their relationship more," she said. "But they hesitate to marry until they feel they have enough economic security to relieve them of the chronic stress that undermines relationships."

As women are becoming more self-reliant, the gap between their pay and men's has been decreasing over the past century.

According to a journal published last year by sociologists Daniel Schneider, Kristen Harknett and Matthew Stimpson, the decline of marriage rates is related to both men's declining economic resources and the rising rate of incarceration.