Dec. 10, 2008 -- Carry -- a Colorado college student who had been in a steady relationship for months -- was recently cajoled by her boyfriend into some sexual experimentation.
He wanted to try anal sex, and even though the 20-year-old said she was "OK with the idea," she nervously downed several drinks before their lovemaking began.
Within 15 seconds, Carry -- not her real name -- said she was "crying and asking him to stop."
They never did it again. But experts say that as social mores ease, more young heterosexuals are engaging in anal sex, a behavior once rarely mentioned in polite circles. And the experimentation, they worry, may be linked to the current increase in sexually transmitted diseases.
Recently, researchers at the Bradley Hasbro Children's Research Center in Rhode Island suggested that anal sex is on the rise among teens and young adults, particularly those who have unprotected vaginal sex.
Experts say girls and young women like Carry are often persuaded to try such sexual behavior for the wrong reasons -- to please a partner, to have sex without the risk of pregnancy or to preserve their virginity. But many don't understand the health consequences.
"It really is shocking how many myths young people have about anal sex," said Judy Kuriansky, a Columbia University professor and author of "Sexuality Education: Past Present and Future."
"They don't think you can get a disease from it because you're not having intercourse," she told ABCNews.com. "They can actually recite by rote how you get AIDS, but it doesn't transfer to their personal behavior."
The study included a comprehensive questionnaire about adolescent sexual and other risk behaviors. The participants self-reported their answers, which scientists say can skew the results in this type of study. To compensate, researchers used audio computer-assisted self -interview technology, allowing participants to enter their responses directly into a computer, rather than having to report to an interviewer.
"Given the subject matter, it is likely that the numbers reported may actually be an underestimate of the prevalence of these behaviors," said Celia Lescano of Brown University, the Bradley Hasbro study's lead author.
More than one-third of new HIV infections in the United States occur among people between the ages of 13 and 29 and can be attributed to the mind-set among youth that they are not at risk of contracting the virus, according to the Kaiser Foundation.
Young Women at Risk
The Centers for Disease Control and Prevention also report that young women, especially those of minority races or ethnicities, are increasingly at risk for HIV infection through heterosexual contact. They are biologically vulnerable, don't recognize their partners' risk factors and are often unequal in relationships.
And when women engage in anal sex, tissue may tear, more readily causing direct blood exposure to infected fluids.
"There is no doubt that teens lack information about STDs and the safety of different behaviors and they they are engaging in more sexual experimentation," Lescano told ABCNews.com.
"That is why studies like these are so important to conduct," she said. "We need to know what teens do and do not know, what behaviors they are engaging in, and what information we need to provide to them so that they can make decisions that will help protect their sexual health."
Even though the topic of anal sex is often considered taboo, Lescano urges "open discussion" of its consequences in doctor's offices, within sexual relationships and with parents.
Anecdotally, young adults confirm the reports of a rise in anal sex, including the perception that it is safe.
"I know some teens who did it just to try, and some who didn't have a condom available," said one 18-year-old Californian who did not want her name used.
"I think that it's less taboo simply because people are more open to trying new things," said her 23-year-old sister. "The younger crowd might be scared by the idea of getting pregnant, so they assume it's safer."
"It takes a great amount of trust to try something new with somebody that you may or may not like," she said. "I'd consider it if there was a ring on my finger. Otherwise, I feel I'd be losing some of my dignity as a woman."
One recent graduate of a New England college said one of her classmates was a "hard-core" Catholic who was rumored to have engaged in risky behavior.
"She only had anal sex with her boyfriend until they were married because that technically kept her a virgin," said the 25-year-old who wanted to remain anonymous.
Indeed, another well-publicized 2005 study using data from the National Longitudinal Study of Adolescent Health found that teenagers who take "virginity pledges" were more likely to engage in oral or anal sex than nonpledging teens and less likely to use condoms once they became sexually active.
STDs Same for Pledgers, Nonpledgers
Conducted by researchers at Yale and Columbia universities, the study found that although teens who made the pledges had sex later than those who had not pledged and had fewer partners overall, both groups had similar rates of sexually transmitted diseases.
In the Hasbro study, females who had anal sex were more likely to be living with their partners, to have two or more sexual partners and to have previously experienced coerced intercourse. Males who engaged in this behavior were more likely to identify themselves as being homosexual, bisexual or undecided.
"These findings suggest that the factors associated with anal intercourse among females in the study relate to the context and power balance of sexual relationships," Lescano said. "We must teach teen girls and young women how to be assertive in sexual relationships, such as refusing unwanted sexual acts and negotiating for safer sex, whether it's anal or vaginal."
Megan Carpentier, who writes about politics and women for Jezebel.com, said girls negotiating with boys for their attention is as old as time.
"Young women are concerned as much about being liked and loved as getting pleasure out of sex," Carpentier, 31, told ABCNews.com. "It certainly was going on when I was in high school: 'Do this if you love me.'"
"Putting it out there -- either oral or anal -- is not a consolation prize," she said. "You are giving up something."
Carpentier said the dismantling of comprehensive sex education programs in the public schools is to blame for teen ignorance about the hazards of anal sex. And it is unrealistic to expect doctors to talk to teens about this touchy topic.
"How many times do teens go to a pediatrician, not a gynecologist?" she said. "And when I was 18, my mother was still in the room."
According to the Journal of Adolescent Health, rising rates of anal sex among teens is reflected in the general adult population where anal sex rates have doubled since 1995.
"Somewhere around 2004-2005 an overused, initially funny and hard-to-attribute quote proclaimed that 'anal sex is the new black,'" wrote sexuality educator Cory Silverberg on About.com.
Pornography Fuels Sexual Interest
Anal sex statistics indicate that a generational change has occurred, where people born in the 1980s and later may be more comfortable admitting to or showing interest in anal sex. Silverberg attributes interest in the behavior among heterosexuals, in part, to the proliferation of pornography in the 1990s.
"I have been having sex for only 12 years, so I don't know if it was just something I didn't talk about when younger," said Tracie Egan, 29, who writes about sex and pop culture for Jezebel.com.
As with other sex trends, girls are more open to experimentation because pornography has become so easily accessible on Web sites like XTube and YouPorn, she said.
"Porn makes people more adventurous with their sex acts," Egan said. "Anal sex is sort of always considered the last frontier, pushing the envelope."
Raised in the 1990s, Egan was exposed to sex-laden MTV, documentaries on gay lifestyles and television shows like "Sex and the City."
"We were raised in a different way," she said. Girls of her generation, the so-called third wave of feminism, she said, were able to have sex with multiple partners and could detach themselves "socially and emotionally."
Still, Egan said that when she had anal sex with a boyfriend for the first time at 26, she was drunk, used no condom -- they were monogamous -- and didn't even know how to clean herself.
"Lack of sex education in school is really bothersome to me," she said. "Even I don't know about a lot of the biological issues."
Columbia's Kuriansky, author of "Generation Sex," has been hearing questions about anal sex from her college students for at least a decade. "Is anything wrong with having anal sex?" is the most common one.
"No," she tells them. "Except if you're forced into it or can't enjoy sex any other way."
Another question she often hears is: "If I want it, or like it, does it mean I'm gay?"
"Not necessarily," she answers.
Girls Now Express Interest in Anal Sex
What's changed in this decade is girls are now expressing an interest. "On college campuses it's escalated," Kuriansky said. "There's more talk, more books, more videos."
One speaker on the college circuit -- Tristan Taormino, author of "The Ultimate Guide to Anal Sex for Women" -- is gaining new ground with young women.
But, Kuriansky said, with fewer educational media outlets and less sex education, young girls are clueless about hygiene, possible bladder or vaginal infections and life-threatening diseases like AIDS.
"We are more open, but there's less information," Kuriansky said. "There are real myths and real efforts to be cool and people running around saying how great it is."
"But it's not just rubbing elbows," she said. "Anal sex is a serious public problem."