No wonder that according to the largest survey on American sexual behavior conducted in decades, published in 2010 in The Journal of Sexual Medicine, researchers at Indiana University found only about a third of girls between 14 and 17 reported masturbating regularly and fewer than half have even tried once. When I asked about the subject, girls would tell me, “I have a boyfriend to do that,” though, in addition to placing their pleasure in someone else’s hands, few had ever climaxed with a partner.

Boys, meanwhile, used masturbating on their own as a reason girls should perform oral sex, which was typically not reciprocated. As one of a group of college sophomores informed me, “Guys will say, ‘A hand job is a man job, a blow job is yo’ job.’ ” The other women nodded their heads in agreement.

Frustrated by such stories, I asked a high school senior how she would feel if guys expected girls to, say, fetch a glass of water from the kitchen whenever they were together yet never (or only grudgingly) offered to do so in return? She burst out laughing. “Well, I guess when you put it that way,” she said.

The rise of oral sex, as well as its demotion to an act less intimate than intercourse, was among the most significant transformations in American sexual behavior during the 20th century. In the 21st, the biggest change appears to be an increase in anal sex. In 1992, 16 percent of women aged 18 to 24 said they had tried anal sex. Today, according to the Indiana University study, 20 percent of women 18 to 19 have, and by ages 20 to 24 it’s up to 40 percent.

A 2014 study of 16- to 18-year-old heterosexuals — and can we just pause a moment to consider just how young that is? — published in a British medical journal found that it was mainly boys who pushed for “fifth base,” approaching it less as a form of intimacy with a partner (who they assumed would both need to be and could be coerced into it) than a competition with other boys. They expected girls to endure the act, which young women in the study consistently reported as painful. Both sexes blamed the girls themselves for the discomfort, calling them “naïve or flawed,” unable to “relax.”

According to Debby Herbenick, director of the Center for Sexual Health Promotion at Indiana University and one of the researchers on its sexual behavior survey, when anal sex is included, 70 percent of women report pain in their sexual encounters. Even when it’s not, about a third of young women experience pain, as opposed to about 5 percent of men. What’s more, according to Sara McClelland, a psychologist at the University of Michigan, college women are more likely than men to use their partner’s physical pleasure as the yardstick for their satisfaction, saying things like “If he’s sexually satisfied, then I’m sexually satisfied.” Men are more likely to measure satisfaction by their own orgasm.