Laura Sessions Stepp is the author of “Unhooked: How Young Women Pursue Sex, Delay Love and Lose at Both.”

At 16, Holly thought she knew the rules of sexual conduct. Sex in high school was something “only those skanky public school girls” did. But once she noticed that her friends at an East Coast private school were doing it, “I had to reevaluate,” she says. Although she abstained through high school, she was fully initiated by the time she was a sophomore in college, even boldly advertising her exploits: “I feel accomplished after I have sex with someone I wanted to have sex with. . . . Good for me.”

Holly is one of more than 70 young women Peggy Orenstein interviewed for her provocative and thoughtful book “Girls & Sex.” Orenstein, author of such books as “Cinderella Ate My Daughter,” here takes a hard look at the sexual landscape young women face today. “Even as girls outnumbered boys in college,” she wonders, did they “have more freedom than their mothers to shape their sexual encounters, more influence and more control within them?” The answer — gleaned from conversations with women and numerous experts — is both yes and no.

Take Holly, for example. Underneath her seemingly proud promiscuity lies a more complex, darker story. When she began college, Holly was, in her words, “very pure.” But after only four days on campus, things began to change. Fueled by social pressures — Greek life, alcohol — she had a series of flings before getting involved in a relationship. When that began to crumble, she got drunk one night and did something she once vowed never to do: have sex outside a relationship. At first she was upset with herself — “I was a bad person” — but soon enough she changed her own rules: Now “I just care that I know the guy,” she tells Orenstein. Later, she slipped even further; one night she drank so much that she didn’t remember the sexual encounter with the guy she woke up next to. “I’ve pushed the line,” Holly acknowledges, searching for a justification: “I don’t know if it’s the culture around me that tells me my behavior is okay, so therefore I’m fine with it, or if it’s because I’m older and more mature and have grown as a person.”

Orenstein refrains from passing judgment on Holly or any of the women she profiles. Instead she turns her eye to the culture that has helped create and foster their behavior. Chief among the negative forces is excessive drinking. In Holly’s case that was three to six drinks a night, Orenstein reports. Among several disturbing tidbits in “Girls & Sex”: Men’s fraternities provide booze and host get-togethers with the 26 sororities and women’s fraternities that make up the National Panhellenic Conference, because the conference does not allow alcoholic drinks in its sorority houses.

"Girls & Sex: Navigating the Complicated New Landscape" by Peggy Orenstein (Harper)

Orenstein also takes issue with marketers of sexy merchandise and the entertainment media for creating ads, TV shows and films that scream sex. She writes, “The impact of garden-variety, ‘pornified’ media on young people — from Maxim magazine to Dolce & Gabbana couture ads to Gossip Girl to multiplayer online games to infinite music videos — is indistinguishable from actual porn.”

Hookups, now well-known as casual sexual encounters, come in for criticism. The practice contributes to the spread of sexual disease and, apparently, has not produced a culture of young women in charge of their own relationships and pleasure. “Teens and young adults account for half of all new STD diagnoses annually,” Orenstein notes. Young women, she adds, engage in some sexual acts not for their own pleasure but to please their partners. “The number one reason they do [oral sex] is to improve their relationships,” according to a study of high schoolers. Nearly a quarter of girls said this, compared with about 5 percent of boys.

Orenstein’s book is both an examination of sexual culture and a guide on how to improve it. In the final chapters, she encourages young women to choose their partners carefully and, if necessary, request or even demand that they receive as well as give pleasure. Girls, like guys, should feel comfortable initiating contact and insisting on reliable contraception. Fortunately for them, there are now 17 methods of contraception, six of which are virtually foolproof when used properly. That’s a benefit their mothers’ generation wishes it had.

Orenstein’s research revealed that young women who attend evangelical churches were less likely to protect against pregnancy and disease, and had the same rates of STDs and pregnancy as non-evangelicals; they also lost their virginity earlier than other women, at 16 on average.

The breadth of Orenstein’s reporting — the variety of experts she consulted, the number of parents she talked to, the science she drew on and of course the girls she interviewed — is impressive. Yet she doesn’t give a full sense of these young women; I wish there was more on what their partners, families and school lives were like.

I also would have liked to see more on fathers. Fathers can play a critical role in building girls’ self-esteem and providing information about the way males think and behave. They also are important in helping boys relate to girls. As Orenstein writes: Boys “need to see models of masculine sexuality that are not grounded in aggression against women, in denigration and conquest. They need to know about shared pleasure, mutuality, reciprocity.”

Sounds like it’s time for a book called “Guys & Sex.”

Girls & Sex
Navigating the Complicated New Landscape

By Peggy Orenstein

Harper. 301 pp. $26.99