It's late on a Thursday night and Jenny is nestled between the sheets of her bed, searching for the perfect video. Phone in hand, she scrolls through clips with titles like, "girls kissing," "girls scissoring," and, "hot lesbians," clicking on and quickly abandoning them when she decides they are not quite what she's looking for. Finally, she settles on a scene featuring dark-haired porn performer Sinn Sage, who kisses, licks, and rubs her pale, blonde partner to climax. As the two women's bodies grind together, Jenny's excitement builds, until finally she too reaches climax, turns off her phone for the night, and falls asleep.

Jenny, 27, identifies as straight, and has been dating her boyfriend, James, for three years. But when it comes to the on-screen images that turn her on, her preference is solidly girl-on-girl. She likes the women it features — "there's something very approachable about them," she notes — and she likes that they seem to try to please each other, rather than one partner just "drilling away" the way that some men in hetero porn films do. Her favorite clips are the ones that feature scissoring, although she doesn't like the fake-looking ones where the women are "on opposite sides of the bed with their legs up," she tells me. "I like it when they're on top of each other, when I can imagine the friction between them."

Jenny is not alone in her partiality to lesbian porn. According to data sourced by Pornhub's research and analysis team, the most popular category among the site's female viewers is "lesbian" (the most popular category for men is "teen"). The girl-on-girl genre is popular among Cosmo readers too. A Cosmopolitan.com survey of 4,000 women published last year found that almost 84 percent of straight female readers have watched lesbian porn at some point in their sexual lives. And 20.3 percent of them, like Jenny, say they prefer it over straight porn. But why?

"I Will Watch Anybody Hump – I Don't Care What Gender They Are."

Female sexual fluidity is a hot topic in popular culture right now — think Piper's back-and-forth between hetero fiancé Larry and ex-girlfriend Alex on Orange Is the New Black, or the plethora of pop stars such as Miley Cyrus, Katy Perry, and Britney Spears, who have played with bisexuality onstage.

"Over the past decade or so, it has become increasingly acceptable — and sometimes even fashionable — for women to express attraction to other women, whether that means hooking up with a friend at a party or seriously dating a woman," observes Shira Tarrant, a professor in gender and sexuality studies at California State University and co-author of the forthcoming book New Views on Pornography.

That's partly a reflection of greater mainstream acceptance of LGBT activism and sexuality. But it's also because sex between two women is seen as sexy and desirable. "Female sexuality and girl-girl sex sell in a way that stories about male bisexuality aren't yet popular," Tarrant explains.

But few scientific studies have researched women's sexual fluidity. In his 2013 book, What Do Women Want?: Adventures in the Science of Female Desire, New York Times journalist Daniel Bergner details one now famous 2007study by Canadian psychologist Meredith Chivers, who measured the genital response of 96 men and women to a range of sexual material — including gay and straight couples having sex, solo masturbation, naked people exercising, and sex between bonobo apes.

The men Chivers measured were aroused primarily by videos that matched their stated sexual orientation. Straight men responded to the videos featuring women, whether they were walking naked along a beach, masturbating, or engaged in sexual intercourse, and gay men responded in the same way to images of men.

But the women's response was determined less by the gender of the person they were watching than by the acts being depicted. A video of a naked man exercising evoked little response in most straight women. But a video of two people having sex elicited a strong genital response, regardless of whether the actors were men or women, straight or gay.

As queer porn producer Courtney Trouble puts it: "I will watch anybody hump. As long as there is a wide shot of people humping, I don't care what gender they are. The movement is what triggers the porn reaction."

"It's Obvious They're Having Fun – Everything They Do Is to Provide Pleasure."

Talk to straight women who watch lesbian porn and they will tell you something similar: that the female performers aren't a barrier to them getting off, but they're not the main attraction either. What really interests them, they say, is that in an industry where so much of what is produced is created by and for men, girl-on-girl porn is the only erotic material they can find that reliably puts female pleasure at its center. "I like lesbian porn because by its very nature, it's focused on what feels good for women," explains Lauren, a straight-identifying25-year-old grad student. "There's a lot of attention to simultaneous vaginal and clitoral stimulation that mirrors the way I get off."

Lauren started watching lesbian porn five years ago, when she grew tired of "scene after scene of aggressive, emotionless penetrative sex" in straight porn. She stumbled upon IFeelMyself.com, an Australian website featuring artfully lit, almost Kinfolk-style videos of female orgasm. "It's mostly girls masturbating, but there's a lot of girl-on-girl porn as well. It felt very real, like it wasn't this fake version of sex that you see in mainstream porn."

Lauren shows me an example of a clip she likes: a 3.5-minute video featuring four young women rolling around on a bed laughing and talking, while three of them bring the fourth one to orgasm by kissing her mouth, penetrating her with their fingers, and touching her breasts and clitoris. "I like that it's obvious they're having fun, and that everything they do is to provide pleasure, not for performance," she explains. "For me, it's not about who is having the sex; it's about imagining myself in the position of the woman who is enjoying herself and feeling her pleasure."

This thirst for porn that feels more authentic is a theme that bisexual porn performer Sovereign Syre, 28 — who mostly performs in girl-on-girl scenes — has heard from her own straight female fans. In porn especially, male-female sexual encounters can often become "power exchanges," she says. "It's difficult for many women to watch a man and a woman having sex without immediately politicizing it." Watching girl-girl porn can be a way for women to step outside that power exchange and enjoy sexual images without the usual cultural baggage.

"It's Not a Contradiction, It's Just a Different Context."

Those "power exchanges" can also impact the way we physically respond to the images we see. According to Kinsey Institute scientists Erick Janssen and John Bancroft, the question of how and why humans become sexually aroused is answered by a "dual control model": a series of exciters and inhibitors (to put it more colloquially, "accelerators" and "brakes") that differ in strength and design from person to person.

Your sexual accelerators are the things that turn you on: a session of witty banter, a great set of abs, or as Courtney Trouble put it, "a wide shot of people humping." Your brakes are the things that stop you from getting turned on — factors whose presence either shut down any potential flicker of arousal (thinking about your parents having sex, for instance) or make it difficult to get aroused in the first place (poor body image, stress at work, or feeling like sex is a task that needs to be completed rather than a pleasure to enjoy).

And in the case of lesbian porn, you've got an equation that — for some straight-identifying women, at least — turns on the accelerator without flicking on the brakes. "Lesbian porn features sex, which is sexy, and it therefore activates your brain's sexual accelerator, just like a lot of other porn does," explains Smith College sex educator Emily Nagoski, author of the forthcoming Come As You Are: The Surprising New Science That Will Transform Your Sex Life. But just as important, it doesn't activate the same inhibitors that straight porn might: by focusing on male pleasure, or by showcasing scenarios in which women are objectified or dominated by men.

Masturbating to lesbian porn doesn't necessarily mean you want to sleep with women, Nagoski says. What switches on our sexual accelerators or activates our brakes depends on context, and "the context of watching porn alone is totally different from the context of having sex with a partner." Women like Jenny and Lauren might be turned on by watching women have sex on-screen, but off-screen, it's sex with their male partners that gets them off. "That's not a contradiction," says Nagoski, "it's just a different context."

Of course, getting off to lesbian porn can be a sign you might be gay. Lindsay, 31, started watching lesbian porn after college, liking that it had less of a "power dynamic" than straight porn and that the acts portrayed mirrored what she enjoyed doing with men.

"It definitely made me think, 'If I'm turned on by watching, I wonder if I would be turned on by the actual doing?'" she says. Now Lindsay identifies as bisexual and has been dating her girlfriend Georgia for a little over a year. "I watched lesbian porn for years before allowing myself to accept that I was attracted to women," she says. "In my mind, being aroused by lesbian porn meant something. And although it did [mean something] for me, it doesn't necessarily for other people."

"Sexual Orientation Isn't as Clear-Cut as We Think."

But the popularity of lesbian porn among straight-identifying women isn't just a matter of it being more "authentic" or better suited to women's tastes. (There's plenty of bad lesbian porn on the Internet, whether it's "barely legal" high school girls or yoga MILFs fantasizing about their husbands while they fuck.) It's also a reflection of a deeper truth: that for a lot of people, labels like "straight," "gay," and even "bi" don't do justice to the complexity of their desires or the scope of their sexual imaginations.

Perhaps the most illuminating research into women's sexuality is that conducted by University of Utah psychologist Lisa Diamond, author of the book Sexual Fluidity: Understanding Women's Love and Desire. Noticing that most studies of LGBTQ experiences focused on gay men, Diamond began a longitudinal study of women's sexual identities in the mid-1990s. She has now been following a group of 89 women for nearly 20 years.

She found that many women's experiences of desire and attraction were more variable than the words they used to describe themselves might suggest. "It wasn't that they were going back and forth between [heterosexuality and homosexuality], but it was totally possible for a lesbian woman to have an occasional attraction to a man," she explains. "There was stability in the overall pattern of attraction — whether a woman was attracted to women, men, or both — but there was also a lot of wiggle room within that."

Diamond's observations on the gap between sexual fantasy and identity are supported by a study published in the Journal of Sexual Medicine in October, surveying more than 1,500 Canadians. It found that 45.2 percent of men have fantasized about having sex with another man, and 36.9 percent of women had fantasized about being with another woman — numbers many magnitudes higher than the 5 percent of the Canadian population who identify as gay, lesbian, or bisexual, according to a 2012 study by the National Post.

That doesn't mean that sexual orientation doesn't exist or that everyone is secretly bisexual, Diamond says. "It just means that [the labels of 'gay' and 'straight'] aren't as deterministic as we think they are," she explains.

For Jenny, even though she identifies as straight, she recognizes that one word doesn't account for all her sexual desires. "The porn I watch is only a small part of who I am and my sexuality," she says. "It's hard to define a person by one thing that arouses them. It's too diverse and complex for that."

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