In a 2012 survey by the National Campaign to Prevent Teen and Unplanned Pregnancy, 87 percent of teenagers said “open, honest” conversations with their parents could help them put off sex and avoid pregnancy. Students who take part in comprehensive sex-ed programs delay having sex for the first time, have less sex and fewer partners and rely more on contraception than their peers. (Conversely, abstinence-only instruction has not succeeded in extending virginity.) “As parents of young children, we are really engaged,” Sorace says. “But sexuality is such a taboo topic in our culture that when it comes to adolescence, we freeze.”

That’s probably why information about sex, whether from parents or schools, is so often delivered in serious, white-coat fashion, its clinical messages heavy with the fear of consequences. To those who advocate abstinence until marriage, attitudes like Metzger’s foster permissiveness. But limiting the conversation to abstinence, Metzger says, “isn’t a full-enough understanding of sexuality.” Because they are voluntary, Great Conversations courses are free to be more frank than school-based sex ed; they can sidestep detractors who think kids shouldn’t be taught about masturbation, for example. “We are not saying you have to learn this,” Metzger says. “People get to choose to come to us.”

Metzger’s greatest challenge might be figuring out how to speak in one voice to families from radically different backgrounds and viewpoints. For the most part, the course, which costs $70, attracts a well-educated, mostly homogeneous demographic. But over the years, Metzger and her business partner, Robert Lehman, who also runs the boys’ curriculum, have tried to appeal to lower-income parents. They found success in Palo Alto, where the class is regularly taught in Spanish. But in Seattle, Metzger says, she has struggled to find a community partner. A deal with the Y.M.C.A. fell through because of the need to simultaneously translate instructors’ rapid-fire delivery into several languages.

Earlier this month, Metzger got an email from a middle-school teacher she knows: Would Great Conversations want to teach a group of disadvantaged students — some homeless, others victims of abuse? Two of her instructors are interested, and Metzger is imagining what shape such a class would take. “It wouldn’t be the same song and dance,” she says.

Metzger’s course might need to evolve in other ways. Lindsey Doe, a clinical sexologist whose YouTube channel, Sexplanations, tackles subjects ranging from kissing to anal sex, attended Great Conversations with her daughter. She was disappointed that the focus was limited to either boys or girls. Where would a transgender or an intersex child fit in? “I loved the curriculum so much that I wanted it to be perfect, and that was the piece that would have completed my experience,” Doe says.

Metzger is open to the idea. Finding the right words to include adoptive families was tricky when she started teaching the course; now, it’s how to deal with sexual identity. “There was a titanic shift five years ago when the audience began demanding a more open conversation around homosexuality and transgender experiences,” she says. “We’re always trying to balance the readiness of the room, and we may be running a bit behind.”

In November, my 10-year-old, Shira, and I attended For Girls Only. There was an undercurrent of nervous tension as we waited for the class to start. Mothers looked stressed, daughters embarrassed. Shira hadn’t wanted to come. “I don’t want to learn about puberty,” she pouted. “I don’t even like the word.” But as the girls looked around, some of them spying friends, they seemed emboldened: Maybe theirs weren’t the only parents to drag them to a talk about penises and vaginas.