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.