For the Left, it has long been as axiomatic as the third law of thermodynamics: Teenagers will have sex, and there’s nothing to be done about it. In the culture wars over sex education, this was always a hotly contested point, but now it has been proven indubitably false by a group of well-placed experts — teenagers themselves.

As The New York Times recently reported, the teen pregnancy rate “has fallen steadily for a decade with little fanfare, to below any level previously reported in the United States.” The percentage of students in ninth through 12th grades who report ever having sexual intercourse has steadily decreased since 1991, according to the Centers for Disease Control. More than half of male high-school students say they are virgins, a substantial increase from 39 percent in 1990.

It is difficult to disentangle the causes of the current trend toward less teenage sexual activity: fear of HIV and other sexually transmitted diseases, awareness of how unwed pregnancies have affected older relatives, a general behavioral conservatism among the latest generation of teens. All play a part.

But another factor has been a cultural shift toward a greater appreciation for abstinence, and an awareness that sex — wonderful as it is — can be a destructive force if not coupled with an emotional commitment greater than the average hormonal boy can muster.

The signals sent by society matter. A decade ago, the United States had a surgeon general, in the person of Joycelyn Elders, who poured scorn on the idea of expecting kids not to have sex. A turning point was the 1996 welfare-reform bill, which put the federal government in the business of promoting abstinence, with a small grant program to the states for abstinence-only sex education. That created a healthy debate about abstinence, and now even the most promiscuity-friendly groups pay lip service to it as a goal.

The fight to keep teenagers from having sex is essentially a fight over whether low-income kids with blighted backgrounds will also have blighted futures. Teenage pregnancy primarily affects low-income girls, who are twice as likely as their better-off peers to get pregnant.

An early debut in sexual activity can create a long-lasting pattern of unstable relationships. Girls who become sexually active at ages 13 or 14 are half as likely to be in stable marriages in their 30s than women who became sexually active in their early 20s. Delaying their age of sexual debut makes it less likely that they’ll be preyed upon by older men — most of the fathers in teen pregnancies are adults — and more likely they will get one of the most important questions in their lives — with whom to have sex and when — right.

The latest trends show that teens aren’t necessarily the problem. We are, in the way we have set expectations so low and told teens it is OK to give away their bodies, and their spirits, so easily.