Now 54, she lives in Berkeley California with her Japanese-American husband. She says she has drifted away from observant Judaism but, after having a daughter, realised that she wanted to imbue her with a strong Jewish identity. "She's just had her batmitzvah - a do-it-yourself affair, which was all about the ceremony, not the party. We made our own siddur and tallit out of old kimono fabric, to show the dual-culture aspect. It ended up being a really meaningful celebration."
Her motivation to write Girls and Sex came partly out of concern for her daughter.
"She was becoming a teen and I was hearing stories about 'hook- up culture' and sexting and binge drinking. At the same time, we were having a larger culture conversation about sexual assault and consent on college campuses. It seemed to me that there needed to be more discussion about what happens after 'yes', particularly for girls. I wanted to know what was going on with them, how they were thinking about sex, what their attitudes and expectations were. I wondered: can you really be equal in the classroom and boardroom if you're not equal in the bedroom?"
Orenstein chose to focus on 15 to 20-year-olds - the age at which most become sexually active - and on girls who were either at college, or college-bound. She conducted interviews with 70 girls from a diverse range of ethnic backgrounds, including Jewish ("although not enough Jews to make any generalisations.").
What was she expecting to learn? "I didn't know. I'd written about the difference between sexuality and sexualisation in my previous books - sexualisation being something imposed on girls from without, and sexuality as a process from within - and how the two were being blurred by popular culture. I was interested in finding out more about that."
What she discovered was a sexual landscape virtually unrecognisable to her generation. "Sometime around the 1990s, the script flipped," she says. "Casual sex used to be the exception; you were in a relationship and then you had intimacy. But with the advent of 'hook-up culture' it's become normalised that sex precedes intimacy, rather than derives from it. Instead of being the first thing you do, dating is now the last."
The term "hooking up" does not necessarily always denote intercourse. Remaining undefined and consequently ambiguous, it can be used to describe anything from kissing to penetrative sex - one reason why girls over-estimate the number of partners their peers have had. "In terms of intercourse, kids are actually not having more partners today, and they're not doing it any younger. What's changed is that they are doing other things like oral sex more. And that for me was the biggest revelation: that somewhere in the late 20th century, oral sex became considered less intimate than intercourse, particularly when it's female to male."
She was disappointed to learn that feminism has not been communicated downwards. While girls today feel entitled to engage in sexual behaviour, they don't feel entitled to enjoy it, measuring their sexual satisfaction in terms of their partner's pleasure, not their own.
"Oral sex, for example, tended to be a one-way street. Girls said they did it because it improved a relationship, or it was a way to curry favour with popular boys, to avoid having intercourse, or even to get out of a threatening situation. I asked girls: "What would you do if, every time you were alone with a guy, he told you to get him a glass of water, but he never offered to get you one? You'd never stand for that. They'd laugh and say, 'Well, if you put it that way…' But why would anyone be less insulted to perform a non-reciprocal sex act than they would be getting a glass of water from the kitchen?"
But it wasn't just that boys didn't want to reciprocate, Orenstein discovered. Girls, too, often didn't want it because of negative feelings about their bodies. "There was a lot of shame around their genitals and they talked about them as though they were simultaneously icky and sacred. Many of them wore skimpy, sexy clothing - which my generation would have seen as self-objectification, but which has been sold as empowerment to today's girls. Yet, a lot of that confidence came off with their clothes."
She blames society for this. "I realised that we perform the equivalent of a psychological clitoridectomy on girls from the time they're babies. We name all boy parts, but with girls we're silent from the navel to the knees. We never say vulva or clitoris, and studies show that fewer of half of girls have ever masturbated. Then they go into their partner encounters and we somehow magically think they're going to be able to express their wants and needs and limits, and think that sex is about them, too - and it's not realistic. We're setting them up for unequal relationships."
Orenstein says our culture may be sex-saturated, but we still don't have open, honest conversations with young women about sexuality and how their bodies should feel to them, rather than how their bodies should look to other people. "Once we stopped saying 'Don't do it,' we did not replace it with discussions about ethics, responsibility, reciprocity and enjoyment. Because of that, the media and now the internet rushed in with a lot of extraordinary, unhealthy messages."
Internet pornography has been a huge game-changer. Worryingly, it's often the first lesson kids get about sex. "If children see extreme images before they even kiss anyone, of course it's going to affect their perception of sex, their sexual script. And porn is another place where female sexuality is there just for male pleasure, as a performance for men. That's why girls say things like: "My boyfriend wants to know why I don't make the noises women in porn make during sex," and also why there is so much pressure to have anal sex."
Interestingly, the "orgasm gap" disappears when it comes to same-sex couples, who do enjoy reciprocal pleasure. Orenstein's conversations with lesbian girls also brought up an issue with the way we define virginity.
"Gay girls can have many sexual partners but never have intercourse, so are they virgins? One lesbian I asked told me she'd decided that she'd lost her virginity the first time she had an orgasm with a partner. Imagine if that were the definition: it recasts sex as a pool of many experiences that involve affection and arousal and desire, as opposed to the traditional race to a goal."
So what can we as parents do? Orenstein believes we should take a lesson from the Dutch, who talk to their children not only about the risks of sex- avoiding pregnancy and disease -but in terms of pleasure and mutual trust and relationships (and have far lower rates of teenage pregnancy). "For some reason, we separate discussion of sex into this 'other' space, when we should really be including it in the lessons we teach our children - girls and boys - about being a mensch, a good friend, not bullying, caring for others.
"We need to teach ethics, reciprocity, pleasure and caring. We need to name our daughters' body parts correctly, and say, "Yes, it feels good to touch your vulva but we don't do it at the Rosh Hashanah table, we do it privately." We have an obligation to look at porn sites to understand what our children might be seeing.
"We are lucky that, as Jewish parents, we come from a tradition that is positive about sex and intimacy. That gives us a great foundation to talk to our kids.
"In our tradition, female sexual pleasure is important," she says. "It's a mitzvah and an obligation to make sure your wife is sexually satisfied, and she can divorce you if she isn't. I've never thought it was an accident that so many prominent feminists are Jewish women, or that I'm a Jewish woman who is concerned about female sexual pleasure."
