We ooh and aah when we see a gorgeous young couple tie the knot these days.

Wedded bliss among the young and the restless, however, seems an increasingly rare occurrence in 2017.

Here are four critical data points and facts culled from research, surveys and polls about how relationships have shifted in recent generations — to help explain why.

1.) Marriage rates for men ages 20 to 39 have declined by about half since the 1970s. In 1970, roughly 70 percent of men in this age group were married — compared with roughly 35 percent today, according to a general social survey. That is quite a difference, to put it mildly.

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2.) Despite the declining rate of marriage for younger males, “married men typically enjoy better physical and mental health than their single peers,” W. Bradford Wilcox and Nicholas H. Wolfinger noted in a piece in National Review in February.

Related: Four Reasons Saving Sex for Marriage Is Worth It   

Wilcox is a senior fellow of the Institute for Family Studies and the director of the National Marriage Project at the University of Virginia; Wolfinger is a professor of family and consumer studies and adjunct professor of sociology at the University of Utah. The two wrote that marriage adds “enormous benefits for men’s wallets, their sex lives, and their physical and mental health.”

“Yet too many men still believe in the ball-and-chain myth, viewing marriage as an expensive encumbrance on their freedom and their sex lives,” Wilcox and Wolfinger wrote.

3.) Moving in with a significant other before marriage is increasingly common. Some 74 percent of all 30-year-old women in America have co-habitated with a romantic partner without being married, according to the CDC. And 65 percent of all couples that eventually get married in the United States have lived together first.

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A 2013 report — also from the CDC — found that 48 percent of all women in America were co-habitating with a partner but not married to them.

“Only 23 percent of the women said they got married first before moving in,” CBS News reported in April 2013.

4.) “Census data from 2012 [showed] that 7.8 million couples [were] living together without walking down the aisle, compared to 2.9 million in 1996,” The Atlantic reported in 2014.

Related: Singer Colton Dixon’s Dating Advice: ‘Don’t Compromise’

“For so long, the link between cohabitation and divorce was one of these great mysteries in research,” Arielle Kuperberg told The Atlantic. She’s an assistant professor of sociology at the University of North Carolina at Greensboro who analyzed data on married individuals. “What I found was that it was the age you settled down with someone, not whether you had a marriage license, that was the biggest indicator of a relationship’s future success.”

Bottom line: Love, dating and marriage — in that order — may be increasingly rare among young people today who emerge from college and spend their time building their careers, living with friends or significant others, and otherwise getting to know the world around them before they even think of settling down.

Pope Francis, in the fall of 2015, took pains to note the trend: He told Congress and the U.S. bishops during his visit to the United States in September 2015 that the culture today is a key contributor to the phenomenon of young people choosing not to get married. In his September 26 address in Philadelphia, the pope said that “we are living in a culture that pushes young people not to form families: some because they don’t have the material resources to realize a wedding, or a life together. But others just choose this because they think they’re better off this way — but that’s the temptation, to not lay a foundation, to not have a family.”