In this July 5, 2015, file photo, a man uses...

In this July 5, 2015, file photo, a man uses the dating app Tinder in New Delhi. Online dating is now hip with young adults, but not always for dating. Mobile dating apps such as Tinder incorporate fun elements and are dead simple to use. That's luring adults ages 18 to 24, who traditionally haven't been big online daters because they are already surrounded by lots of other young, single people. (AP Photo/Tsering Topgyal, File) Credit: AP

Millennials are less sexually active as young adults than previous generations were. On the surface, that looks great: They appear to be less disposed toward risky behaviors, better at saying no to unwanted encounters, more motivated to study, work and make money, which could lead to more financially secure, happier families. Yet there could be an ugly side to this that could turn what looks like increased responsibility into a demographic threat.

According to a paper by Jean Twenge of San Diego State University and her colleagues Ryna Sherman and Brooke Wells, published on Tuesday in the journal Archives of Sexual Behavior, 15 percent of 20- to 24-year-old Americans born in the 1990s have had no sexual partners after their 18th birthday, compared with just 6 percent of people born in the 1960s at the same age.

This is in line with previous research showing that those millennials who do have sex tend to have less or it and fewer partners. And when they do hook up, in most cases they have the kind of sex that Bill Clinton memorably refused to recognize as such: according to a recent study by Arielle Kuperberg of the University of North Carolina at Greensboro, only a minority of U.S. college undergraduates say they had penetrative sex during their most recent hook-up.

It’s tempting to see this as harmless. After all, an early sexual initiation increases the risk of teen pregnancies (which are quickly becoming less common) and abortions, and it may lead to riskier and less happy sex lives later on as well as high risk of sexually transmitted diseases. Let kids study and avoid cumbersome, disruptive relationships — the reasons most often cited by millennials when they are asked about their low sexual activity levels — until the reproductive urge finally catches up with them at a more mature age.

It’s also nice that young women are reportedly happier with their early sexual encounters: The continued prominence of alcohol-induced sexual encounters and date rape on college campuses notwithstanding, there’s less unreported sexual violence and reluctant acquiescence because women are more confident than ever before — and because porn is an easy way for young men to channel those fantasies. There’s also less stigma attached to having premarital sex.

The growing acceptance of sex under any circumstances and in any form is a rather paradoxical companion to the lower sexual activity, Twenge points out in her paper. Perhaps, she suggests, this is evidence of a "rising individualism wherein individuals hold permissive attitudes about a variety of behaviors while also feeling less pressure to conform in their own behavior." That sounds healthy, too — but the formation of families has long been a matter of social norm as well as individual choice. In a 1972 paper, Gudmund Hernes described the "social pressure to marry" as self-evident: "We all know this pressure increases with the increase in the percent of a cohort already married."

Bergen’s paper makes for slightly comical reading today: It’s rife with 20th-century stereotypes about singles being invited to parties less often as they get older because they can be a threat to existing couples, or about a popular culture that describes a woman as a "spinster or even reject" unless she marries by a certain age. Millennials’ rejection of this kind of social conformity is the current mainstream, but it’s too early to say whether the disappearance of the "social pressure to marry" will be good for birth rates. Even now, they are higher in countries with traditional, collectivist cultures, where the pressure hasn’t disappeared.

The current sexual culture also has different rejection mechanisms that are perhaps more cruel than the old-school ones. Twenge wrote:

"New technology may have created unequal outcomes.While some young adults may use apps such as Tinder to hook up with many partners, a growing minority may be excluded from this system entirely, perhaps due to the premium placed on physical appearance on dating websites."

That, more than any kind of newly ingrained risk aversion or responsibility, may well explain the higher sexual inactivity rates. People who might turn out to be quite attractive on a real-world date don’t even get a chance to go out with anyone because their picture on a dating app is not particularly flashy. It’s curious that, as the Twenge paper points out, sexual inactivity rates haven’t increased for college undergraduates: In college, people don’t need an app to start talking to each other and eventually dating.

The perceptions of attractiveness — and some of the sexual practices followed when young people do become intimate — are probably shaped by porn to a greater degree than we realize. There have been plenty of warnings about it "hijacking our sexuality," but countries have mostly chosen not to regulate it — only child pornography is universally banned, and there’s little research into how it affects real-life sexual habits.

Congratulating today’s kids on being more responsible is probably not the best possible reaction to the crawling sexual counterrevolution. Sweden’s Health Minister Gabriel Wilkstroem, for one, is concerned about the growing evidence that Swedes are having less sex. He wrote in a column for the daily Dagens Nyheter last week:

"The problem is that as long as we only focus on the negative aspects without also highlighting the positive and pleasurable side of sex, we will never really be able to solve the problems. How can we prevent a distorted view of women, often shaped by women-degrading pornography, and even sexual harassment or rape if we can’t present alternative images of how sex and relationships between people could be?"

Wilkstroem is launching a detailed government-funded study of Swedes’ sex lives to figure out how best to help people understand that sex isn’t just something to be careful about. Like most of the Western world, Sweden has a birthrate of less than two children per woman (propped up by much higher birth rates among the country’s growing Muslim community), and it might like young people to be a little more irresponsible, as they used to be in an age before Tinder and ubiquitous online porn.

Leonid Bershidsky, a Bloomberg View contributor, is a Berlin-based writer.