Katherine, 25, grew up in a conservative Pennsylvania town and went to schools where the only sex education was abstinence-based. Her parents didn’t raise her with any strict views on sex before marriage, or with the sense that it was a bad thing. Instead, sex simply wasn’t discussed in her family — ever. Despite her upbringing, Katherine grew into a self-described progressive, extremely liberal and sex-positive woman.

A liberal, sex-positive woman who didn’t lose her virginity until she was 24 years old.

In high school, Katherine, who asked to be identified with a pseudonym, was shy and didn’t get much attention from boys. She had her first kiss during her freshman year of college and later had a few “intense make-out sessions,” some of which included oral sex, but generally felt herself lagging behind her peers sexually. Then last year, fueled by alcohol, Katherine kissed a close male friend. She wasn’t necessarily interested in him romantically, but they had good chemistry and spent the subsequent week talking about what it might mean for them to have sex. Katherine was open about her history, and he knew she was a virgin. She felt comfortable with him and thought, If not now, when?

“I got to the point where it felt like if I didn’t take this particular opportunity, it was like, ‘What am I waiting for?’” Katherine said.

“You get to a certain age and then it becomes this weird elephant in the room of, ‘Well, why haven’t you had sex yet?’” she continued. “‘What’s your hang-up?’”

The average age at which American women lose their virginity — which, for the purposes of this story, is defined as having vaginal intercourse — is 17. For many virgins, abstinence is a deliberate, ideological choice: Nearly 40 percent of teenage girls who have not had sex say that it goes against their religious or moral code, while others say it is because they don’t want to get pregnant or haven’t met the right person yet.

But many other women who stay virgins into their 20s and 30s don’t cite any specific reason for doing so. It’s simply how things shook out.

These “unintentional” virgins are part of a group that is often overlooked, even by sex researchers — they’re not necessarily kindred spirits with women who eschew premarital sex on strong religious or moral grounds, but they can’t quite relate to their sexually active peers.

Throughout most of Western history, virginity has been a prized virtue for women to protect, said Hanne Blank, a historian and author of Virgin: The Untouched History. It wasn’t until the early 20th century, when women began to leave their homes to take jobs in factories, that this idea began to slowly shift. And with the introduction of the birth control pill in 1960, broader cultural norms swung dramatically, Blank said. By the so-called “summer of love” in 1967, it was all but expected that American women without religious reservations would have sex in their teens or early 20s — an expectation she believes endures today.

“We may not still have this thing where if you’re not married by the time you’re 25, you’re a colossal failure and an old maid and all of that, but we definitely have a very similar cultural rhetoric where if [you’re a virgin it means] nobody has found you sexually desirable by that point and something is wrong with you,” she said.

Dawn, 25, who also asked to use a pseudonym, was a virgin until last summer. She is all too familiar with the feeling that her virginity made her an aberration of some kind — a “unicorn,” as she put it. In high school, she just wasn’t particularly interested in boys, and in college she was very focused on her classes.

“I was busy being awesome academically,” Dawn wrote in an email to HuffPost. “Not having sex yet at that point didn’t seem like it was such a big deal.”

But after college, perceptions began to shift. Friends assumed she’d had sex, and those she corrected readily offered often unwanted commentary.

“My girlfriends thought it was intentional and thought I was so sweet for waiting,” Dawn wrote. “My guy friends asked if I was religious and advised not sharing my virginal status with guys I was interested in, if at all, until they were already ‘hooked’ (whatever that means).” Some friends suggested she go out, get wasted and “get it over with” with a willing stranger.

The men she dated had varied, but equally strong, responses. Some were scared to have sex with her because they thought it would be too much responsibility, while others were “grossly curious” about what it would be like to “deflower” her. “They were not interested in having sex with me the person, but with a virgin,” Dawn said.

All of it made her feel very alone.

“Frankly, we’re damned if we do too soon or too many, and damned if we don’t soon enough or wait for the right person,” Dawn wrote. “I really feel like society needs to catch up and be more supportive of the many paths people end up on.”

Modern cultural representations of 20-something virgins are rare, and tend to follow a similar narrative. Shoshanna on HBO’s “Girls” keeps her virginity a secret for fear of scorn, and is at one point turned down by a prospective partner who tells her, “It’s just like virgins get attached. Or they bleed. You get attached when you bleed.” Late-in-life virginity can also be fetishized, as was recently the case on “The Bachelor.” A 27-year-old contestant openly worried about how her virginity would be perceived, only to be told by another contestant, “He will like it! Every guy likes it. Guys like taking your virginity.”

Other “unintentional” virgins find that their virginity becomes more important to them as they get older.

In high school, Jenna, 26, who identifies as Christian, made a decision to “save herself” for marriage, but in college that commitment began to fade. If she had found the right person then, it’s likely she would have had sex. But it just didn’t happen.

To date, Jenna has kissed a few people, but nothing more. She describes herself as in tune with her body and her desires. She began masturbating in college, and feels confident that when the time comes to have sex, she’ll be comfortable showing her partner what she enjoys and what turns her on — something she is not sure she could have done when she was younger. And after waiting so long, she is now fully recommitted to the idea of waiting until marriage.

“At this point, I definitely want the decision to be on my own terms,” she said.

For Nacole, 33, waiting until marriage isn’t necessary, but waiting for the right moment absolutely is. For years, she had crushes on boys who told her they thought of her as just a friend — a fact she attributes at least in part to being one of the few black girls growing up in a mostly white area. In high school she was open to having sex, but no one asked.

Men hit on Nacole as she got older, but she didn’t want her first time to be with just anybody. Then last fall, she started dating a man who pursued her diligently, sending her secret-admirer emails. He has become her “first honest-to-goodness boyfriend,” Nacole said, and she has been open with him from the start about the fact that she is a virgin. He, in turn, has frequently praised her for being “strong enough” to make it into her 30s without having sex. “I’ve had to remind him, ‘No, no, no, it’s not because of me,’” she laughed.

With each step they take together sexually, Nacole feels a mix of happiness — she’s glad she waited for him and that she feels mature enough to give herself fully to the experience — and goofy nerves.

“Every time something happens that goes to that next level physically, I lose it, freak out, then get all anxious and excited and laugh,” she said. “Thankfully, he thinks that’s really endearing.”

Nacole wants to sleep with her boyfriend, but at this point in her life, she feels no sense of urgency — for the most part.

“Wanting to do this ‘right’ and not make a mistake at the last yard line now feels important to me,” she said. “That’s probably 70 percent of me. Then there’s the 30 percent that is like, ‘This is not a big deal, you’re just one person in a world of a billion, no one cares. It’s fine, just do it and get it over with. Join the club.’”