It can be lonely in Las Vegas — so lonely, a single 23-year-old woman is looking for a new bestie on Craigslist.

"There are guys all over the place to have fun with," she laments in an anonymous post, "but I wish I had a friend to go shopping and to concerts with me or to get our nails done ... someone I can talk to about my boy problems, maybe hear about hers."

Oh, and one more thing!

After rocking out at Britney's live show, she's also hoping she and her new BFF will get in bed and go down on each other.

The post continues: "I left out the part about not being able to keep our hands off each other when we're alone, giving each other hot and steamy kisses, and seeing what we taste like."

Wait, what?

She's one of thousands of women across the country, judging by Craigslist and other sites, who identify as straight — explicitly mentioning boyfriends, husbands, or dating dudes in personal ads — but are also looking online for casual, lesbian sex. It's more than experimenting with your college suitemate, and it's not about turning on bros at the bar. These hetero flexible women say they want long-term romantic relationships with men but are consistently drawn to women purely for sex, whether a girlfriends-with-benefits scenario or a Sapphic sidepiece hidden from their boyfriend or husband.

"It's a huge phenomenon," says Chelsea Reynolds, who researches gender and sexuality in mass media as a doctoral student at the University of Minnesota School of Journalism and Communication. In a pilot study, she analyzed (or "expertly lurked," as she calls it) the Women Seeking Women section of Craigslist in 10 U.S. cities, including San Francisco; Chicago; Boston; Louisville, Kentucky; and Lubbock, Texas. Over a three-day period, she analyzed more than 400 such ads. Based on that data, she estimates that, over the course of a year, there are hundreds of thousands of ads posted online from women who "self-identify as straight, who want relationships with guys, but also enjoy a woman's body and affection here and there."

When straight girls shop on the Web for gay sex, Reynolds found that they maintain their "stereotypically heterosexual" (read: basic bitch) identities. They reference the guys in their lives, most of whom are in the dark. "My husband will not know or join," clarifies a 30-year-old from Austin looking for "the touch of a woman" on Craigslist. Many ads toss out invites for girly activities like mani-pedis and brunch. "I want to hang out and go shopping and be friends outside of sex," explains a 24-year-old woman from Evanston, Illinois, in her Craigslist callout.

Rocco Bizzarri/ thelicensingproject.com

Many are clear in their online ads that they're looking for "femme, not butch" women, because it's precisely the feminine "lipstick" ladies who offer them a sexual break from the men in their lives.

"Women are so soft and captivating," gushes Ruby, a 31-year-old fashion sales coordinator who lives with her boyfriend in New York but has a long-standing lust for women that she's acted on in the past. "They're like gummy candy in my mouth."

Or as one Dallas woman recently put it on Craigslist:

"Hey, ladies, I'm 36 and married to a douche. I'm longing for a pretty lady to give me butterflies and orgasms."

Reynolds herself is in a monogamous relationship with a man but says she's attracted to women and the way they seem to innately know how to please her. "That doesn't necessarily make you gay," she says. "The way I've explained it to my partner is 'I'm super into you. I love having sex with you. But at the same time, there's one thing that you don't have, which is a vagina.'"

About 7 percent of women in the U.S. identify as gay, lesbian, or bisexual, according to Indiana University's 2014 National Survey of Sexual Health and Behavior, which polled 5,000 adults. But the number who have had same-sex hookups at some point in their lives is higher — closer to 12 percent, according to the same data. The numbers are highest among women 25 to 29. Nineteen percent in that age group have had oral sex with a woman.

The idea that a person can be kinda-sorta straight or a lil bit gay goes back almost 70 years to pioneering researcher Alfred Kinsey. He argued that some people are straight, others are gay, but many others fall on a spectrum somewhere in between. Still, the idea that sexuality is fluid is blossoming in the mainstream of late. Orange Is the New Black and its "gay for the stay" jailbirds are Netflix magic. Breathless rumors swirl that Taylor Swift and model BFF Karlie Kloss are makeout buddies (a claim they laugh off). A survey by Cosmopolitan.com of 4,000 women published last year found that almost 84 percent of straight female readers have watched lesbian porn.

The number of women who describe themselves as mostly but not completely heterosexual is on the rise, according to the most recent National Health Statistics Report. In 2008, 12 percent of women in the U.S. said they were "mostly" straight but have some attraction to other women, up from 10 percent in 2002. These semi straight women likely always existed, says Lisa Diamond, Ph.D., a professor of developmental psychology at the University of Utah and author of Sexual Fluidity: Understanding Women's Love and Desire, but horny hetero flexible 1950s housewives couldn't exactly shout it across the white picket fences of the era.

In 1995, Diamond began tracking the sexual orientations of a group of 100 women ages 16 to 23. After re-interviewing them every two years (she's currently prepping for her 20-year check-in with the group, which now consists of 89 women), she found that 72 percent reported "wiggle room" around traditional labels like gay, straight, and bi. "There are lesbians who were like, 'I got involved with my best male friend,' or heterosexual women who were like, 'Well, I started fooling around with my best female friend,'" she says. "As the years went by, it was clear that these were not exceptions but pretty common behaviors."

Often, she's found that women aren't trolling for random girls at bars but falling into bed with "a close hetero-sexual friend" because "their feelings are so strong emotionally that they kind of spill over into the physical realm."

The first girl who Allie, a 31-year-old artist from Kentucky, hooked up with was a longtime friend on a trip to the friend's family cabin. The next time, she was wooed by a fellow bridesmaid in a mutual friend's wedding at a lakehouse bachelorette party. "We ended up having sex, and it was awesome," says Allie, who notes that the bride never found out nor did Allie's boyfriend at the time.

So if "straight" women are into lesbian sex, how are they not lesbians? Or bi? Diamond, who is a lesbian, admits that in the early stages of her study, she was waiting for the "wiggling" women to come out eventually one way or the other. Instead, year after year, the women said things like, "That was an amazingly powerful relationship with Anna, but I'm pretty sure I'm still straight."

Many straight women who occasionally sex other women insist they're not lesbians — because they only want to date men. And they're not bi either; they don't want to date both genders seriously.

Ruby says her magnetic attraction to women has made her question whether she's a lesbian. But ultimately, she identifies as straight. "If I had a relationship with a female, wouldn't it be like having your bestie over all the time? Could I take it seriously?"

For some, like Nicole, a 31-year-old journalist in Cleveland who says she's "one hundred percent straight," hooking up with other women in no-strings-attached situations is a way to explore what turns her on in a safe, no-boys-allowed kind of environment.

"When I hook up with men, it's because I want to date them, so there's a level of awareness [during sex] in wanting things to work out," she says.

With girls, "the pressure's off. It's like, 'Oh we have the same body parts ... let's make each other feel good for a little bit, then I'll learn more about what I like too.'" This never backfires — except when it does. "There's fear among some lesbian women that [straight yet fluid] women are just going to mess around with them and break their hearts," says Trish Bendix, the editor-in-chief of AfterEllen.com, a popular lesbian-focused website. She adds that many younger members of the lesbian community support women's right to "love who they love," but there are also concerns that fluid women are down for gay sex, but not the struggle that can come with being gay or bi: "I hear from a lot of women who want others to identify and be on our team."

Allie says she has unintentionally confused a few female hookups. Some want to know what she is — a lesbian? Bisexual? "I like a person and I'm attracted to a person. Sometimes that person is a man, and sometimes that person is a woman; sometimes that person is short, and sometimes that person is tall," she says. "I guess I don't label myself because I don't want others to label me either."

In the future, sexual labels may die altogether, Diamond predicts. In the early '90s, the idea of being unlabeled didn't even exist. "If you didn't have a sexual identity label, it meant that you hadn't come out yet," she says. Now, young people are much more likely than in previous decades to come out as unidentified, says Diamond. "To say, 'As long as I love and accept myself, I don't really care what you call me.' I think that might be, in some contexts, the healthiest way to be."

This article was originally published as "Seeking: A Bestie Who Loves Brunch, Netflix Binges, And ... Going Down On Each Other." in the May 2015 issue of Cosmopolitan. Click here to get the issue in the iTunes store!

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