When I hear the phrase “friends with benefits,” I think about my friends who have fabulous beach houses or box seats at the U.S. Open.

That’s not exactly what pops into the head of a 16-year-old boy or girl these days upon hearing that phrase.

“Having friends with benefits is not such a big deal,” a uniformed 17-year-old girl told me as we waited to pay for iced coffees next to each other at Jackson Hole, the famous burger joint that is frequented by the skirted girls of Spence, Nightingale, and Sacred Heart. “All the parents are so freaked out about their kids hooking up, but it’s just normal to us. Very few of my friends want serious relationships.”

Let me be a little more specific: According to a study conducted last year by the National Center for Health Statistics, slightly more than half of American teenagers between ages 15 and 19 have engaged in oral sex. And according to a 2004 study conducted by the Manhattan Institute, 40% of urban 12th-graders have had sex with a person with whom they did not have a romantic relationship.

The teenagers I spoke with uniformly confirmed that in their minds there is a big difference between intercourse and oral sex. “I don’t know so many people who’ve actually had sex with people they’ve just met,” a 16-year-old boy who attends a co-ed private school in Riverdale told me. “But people definitely meet at parties and hook up.”

Hook up?

“You know, fool around,” a 15-year-old girl who attends a private school on the Upper East Side told me.

“Have sex?” I asked.

“Not necessarily sex. But you know, fool around,” she repeated.

“Can you be specific?” I asked.

“It can mean anything, really,” she said. “It doesn’t mean sex. But most of the time it means heavy-duty fooling around.”

The NCHS report found that a quarter of all 15-year-old girls have engaged in oral sex, and more than half of all 17-year-old girls have. Perhaps even more surprising is the fact that the researchers found that females and males are just as likely to be on the receiving end.

“Can we call that progress?” a mother of two teenagers asked wryly. “The truth is that kids know about all the consequences of sex. They think sex is a big deal. But oral sex, which in my day was almost more intimate than sex, is the opposite. It’s casual. And that’s hard for me and my friends to accept. But each generation has its norms and as parents we’ve got to stay current. Just as I stated the facts about sex for my kids, I forced myself to state the facts about oral sex.”

“Most of my friends think that having sex is a big deal,” the 15-year-old girl said. “Although some of my friends who are a couple years older than me — the ones who’ve already had sex — they’re more likely to do it pretty casually. But I know some people who are still virgins who’ve had threesomes and done all sorts of other stuff. Just not sex.” She’s right: According to the studies, nearly one in four virgin teenagers has engaged in oral sex.

An uptown pediatrician who specialized in adolescent medicine confirmed the distinction between sex and anything and everything but sex. “I’d estimate that more than a third of the teenagers I see have had sex,” she said. “But more than half — maybe even 60% or 70% — have engaged in oral sex. And just because kids haven’t had sex doesn’t mean they haven’t had oral sex. They’re viewed as entirely different entities. And teenagers today are pretty adventurous and willing to experiment in all sorts of ways.”

She added, “We’ve done a good job educating them about the risk of HIV and pregnancy and other sexually transmitted diseases. But we’ve missed the ball when it comes to teaching them that there are risks associated with oral sex.”

In the past couple years there’s been more than one sex scandal among eighth graders at private schools in Riverdale and the nearby leafy suburbs. At least two of them involved teenagers e-mailing each other videos of themselves masturbating with various objects. When I ran into a board member of one suburban school recently, he said he couldn’t really remember the details of the scandal at his school. But not everyone forgot so quickly: All three of the involved students are attending other schools.

That doesn’t mean every parent should panic. As the pediatrician said, “Some 13- and 14-year-olds are exactly the way you imagine a 13- and 14-year-old: curious and inquiring and still pretty innocent.” But they should at least be aware.” By and large I’ve found that parents of youngish teenagers have no idea what their kids are up to,” she said. “Parents of 18-year-olds know that their kids might be having sex. But parents of 14-year-olds? They don’t know and by and large they don’t want to know.”