These should be boom times for sex.

The share of Americans who say sex between unmarried adults is “not wrong at all” is at an all-time high. New cases of HIV are at an all-time low. Most women can—at last—get birth control for free, and the morning-after pill without a prescription.

If hookups are your thing, Grindr and Tinder offer the prospect of casual sex within the hour. The phrase If something exists, there is porn of it used to be a clever internet meme; now it’s a truism. BDSM plays at the local multiplex—but why bother going? Sex is portrayed, often graphically and sometimes gorgeously, on prime-time cable. Sexting is, statistically speaking, normal.

Polyamory is a household word. Shame-laden terms like perversion have given way to cheerful-sounding ones like kink. Anal sex has gone from final taboo to “fifth base”—Teen Vogue (yes, Teen Vogue) even ran a guide to it. With the exception of perhaps incest and bestiality—and of course nonconsensual sex more generally—our culture has never been more tolerant of sex in just about every permutation.

But despite all this, American teenagers and young adults are having less sex.

To the relief of many parents, educators, and clergy members who care about the health and well-being of young people, teens are launching their sex lives later. From 1991 to 2017, the Centers for Disease Control and Prevention’s Youth Risk Behavior Survey finds, the percentage of high-school students who’d had intercourse dropped from 54 to 40 percent. In other words, in the space of a generation, sex has gone from something most high-school students have experienced to something most haven’t. (And no, they aren’t having oral sex instead—that rate hasn’t changed much.)