"Honestly, it's been like a honeymoon the whole time," she says softly, speaking to me from her home in the northwestern US. (She won't reveal which state, exactly, fearing legal repercussions.) Katherine speaks slowly and measuredly, with a very slight Southern accent, though her cadence increases whenever she mentions Scott. In the background of the call, there's an almost unnervingly idyllic chorus of birds chirping.

Though she was elated that Scott felt the same way, Katherine felt apprehensive, for obvious reasons. "We started questioning, are we normal? Is this something that happens to a lot of people who are adopted, or is this just something wrong with us?" Katherine recalls. "You wonder, because it's such a unique situation to begin with."

Within two weeks, the conversation turned to more intimate topics, and Katherine and Scott discovered a mutual interest in BDSM. Katherine sent Scott a link to her profile on a fetish site, making sure to upload some alluring photos of herself—"I wouldn't say X-rated, but, you know, sexy," she says—beforehand. After that exchange, Scott admitted that he'd been "having thoughts," and the two finally confessed their feelings to one another.

After three days of correspondence—and after they'd exchanged birth certificates, thus proving their blood relation to one another—they decided to exchange photos as well. According to Katherine, Scott bore a striking resemblance to her, which intrigued her even more. "When we swapped pictures, it was like a ton of bricks hit me. It was like looking at myself in a male version," she says. "I was attracted to him immediately, but I didn't know if he was attracted to me or not."

But Scott was different. After he recognized her last name and friend-requested her on Facebook, the two began messaging, and Katherine immediately started noticing similarities. "Same interests, similar thought processes," she recalls. "We liked a lot of the same things: favorite color, stuff we liked to eat—just, you know, general things."

Katherine and Scott met the way many couples do: online. Katherine, who was 32 at the time, had created a Facebook account using the last name of her late biological father, whom she had never met. Put up for adoption at a young age, she had been trying to track down her various biological relatives since she turned 18. Though she'd made contact with a few of her half-siblings over the years, none of their relationships had deepened or endured. Even when Katherine located family members, she never really felt the genuine connection she'd worked so hard to find.

"GSA happens when blood relatives meet up again, and, differently from meeting a total stranger, there is an immediate history, sense of safety, and intimacy due to shared blood and [kin]," explains Dr. Marlene Wasserman, an international clinical sexologist and author of several books on women's sexuality. "This is enormously comforting and appealing."

The term "genetic sexual attraction" has since made its way into the Oxford Dictionary of Psychology, where it's defined as "erotic feelings between close relatives, often between siblings or between parents and children, who are separated early in life and reunited in adolescence or adulthood."

In her memoir, I'm His Mother, But He's Not My Son, Gonyo writes that she was 42 and married, with three other children and a six-month-old grandchild, when she first met Mitch. Though Mitch didn't reciprocate her feelings, Gonyo was deeply affected by her feelings of attraction towards him. She went on to become the public face of GSA, publishing numerous other books on her experience and becoming deeply involved with a support group called "Truth Seekers in Adoption."

The term "genetic sexual attraction" (GSA) was popularized in the late 1980s by Barbara Gonyo, an American woman who claimed she had fallen in love with her biological son, Mitch, after reuniting with him 26 years after she first put him up for adoption.

After a cursory Google search, Katherine learned there was a name for what she and Scott were going through: genetic sexual attraction.

Wasserman cites the Westermarck effect, a "hypothetical psychological effect" often used to explain the phenomenon of genetic sexual attraction. According to Dr. Wasserman, this theory holds that "people who live in close domestic proximity during the first few years of their lives together become desensitized to any later sexual attraction." In GSA relationships, the belief goes, the Westermarck effect is absent—which explains the feelings of uncontrollable attraction some reunited relatives claim to struggle with.

As a media trope, incest is arguably only just now gaining mainstream visibility. In 2014, following the break-out success of Game of Thrones, which features a brother and sister involved in a consensual incestuous relationship, as well as the Lifetime remake of Flowers in the Attic, which contains the same plot line but without dragons, several media outlets dubbed incest a trend.

Within two years, Katherine had moved across the country to live with her half-brother; they met in person for the first time in the airport. "We were grinning like kids in a candy store," she recalls. "I saw his eyes—they were my eyes as well. We hugged a long time, and I finally felt, for the first time in my life, that I was where I was supposed to be." On the car ride home, they pulled over and shared their first kiss.

Of course, media fixation is vastly different from social acceptance, and it wasn't until Katherine and Scott learned of others struggling with genetic sexual attraction that they felt comfortable with their situation. After reading extensively about GSA, they decided their relationship was neither abnormal nor wrong. "Sometimes it's reciprocated and sometimes it's not, but it's perfectly normal when you haven't grown up with that person," Katherine insists. "Knowing that, we felt a lot better about how we felt about each other, so we just kind of decided to let nature run its course."

"It feels like incest is really in right now," reads a listicle on the Huffington Post, entitled "13 Incestuous Pop Culture Couples with Cringe-Worthy Chemistry." A similar post on the AV Club—this one featuring a staggering 21 couples comprised of siblings—declares that "fall's latest trend" is "making brothers and sisters kiss." The data support this assertion: According to a report from leading adult content providers GameLink.com, released earlier this year, there was a 178 percent increase in the consumption of "family role-play porn" between October 2014 and January 2015.

Katherine runs a site called Lilys Gardener [sic]; the name refers to Lily Beckett, the protagonist of Love's Forbidden Flower , a romance novel about a brother and sister engaged in an incestuous love affair. "The right to be with whomever you choose as long as all are consenting adults should be a basic human right," the homepage reads.

In much of America , sleeping with one's sibling is punishable by prison time. However, a growing community of people like Katherine and Scott—many of whom were adopted at a young age and reunited later in life—say that so-called "consensual incest" is a victimless crime and should be treated as such. On mostly anonymous forums and blogs, some of those who have experienced GSA advocate for the abolishment of anti-incest laws, arguing that sexual relationships between relatives are essentially no different than any other type of sexual relationship, as long as they take place between consenting adults.

Her fears are legitimate. In 1997, for instance, a brother and sister named Allen and Patricia Muth were sentenced to several years in separate maximum-security prisons after being convicted of incest in Wisconsin. At the time of their arrest, Patricia was 30 and Allen was 45; when they first met, Patricia, who had been raised in foster care, was 18. Allen Muth later appealed the case, arguing that he and his sister were both consenting adults. In 2005, a federal court ruled against him , noting that incest is "a condition universally subject to criminal prohibitions."

"It's kind of sad when we go out in public. I'd like to be able to hold his hand sometimes or be able to give him a kiss when I'm happy," says Katherine. "That's very sad to me, because we did not grow up together. If I had met him on the street and not known we were related, I think I would have still been attracted to him. It's very sad."

Katherine and Scott's friends and family members think that they're merely living together as siblings. They're conscientious about keeping up the ruse—they never touch one another outside the house, and they keep their curtains drawn when they're home alone together. It's not simply because of the social taboo: According to Katherine, she and Scott could face up to 15 years in jail if they were discovered and prosecuted.

"In some instances, being related actually makes you a bit closer," she says. "Because you love them as a family member, and you also love them as a partner, a soul mate, the love of your life that everybody dreams of having."

Modern incest laws are rooted in protecting the integrity of the family unit: firstly, by preventing the proliferation of congenital abnormalities, which are more likely to occur among children of close relatives, and secondly, by protecting children from sexually abusive parents and older relatives. As recently as the 19th century, incest was treated as a consensual crime, in which both parties were held equally responsible and were both subject to punishment. In the 70s and 80s, however, second-wave feminists organized to reclassify incest as an act of sexual abuse.

Other experts disagree. "As a therapist, I'm opposed to the decriminalization of incest," says Dr. Wasserman. "Hierarchies of power and influence exist in all families, and when these are distorted through sexuality and intimacy, much harm happens to people—especially those who are in lower rankings of power."

Even some outside the online community have argued that incest should not be forbidden by the state. Thomas Søbirk Petersen, for instance, is a Danish professor of criminal justice ethics who has publicly advocated for the legalization of sibling incest. "If we accept that sex between consenting and rational adults is morally acceptable, then we should also accept sex between consenting and rational adults that happen to be siblings," Petersen tells Broadly over email.

"People link incest to other sorts of things, such as abuse and pedophilia," says Katherine, "but we already have laws for that. A child can't consent to sex, so there's already a law for pedophilia. It shouldn't matter whether [the people in the relationship are] related or not. The fact is, if they're not of age, it's wrong to begin with. Children cannot consent, but adults can.

Like Petersen, many in the GSA community vehemently differentiate between rape or child abuse and what they describe as sexual relationships between two consenting adults who are blood-related.

If I can consent to have sex with an entire football team, why can't I consent to have sex with my brother who is over the age of 30?

But Petersen argues that we already have laws to protect people from sexual assault and coercion—regardless of whether they're related. "Nobody is really hurt" in cases of consensual sibling incest, he insists, and he's not alone in this view. In 2014, Germany's national ethics council called for the decriminalization of incest between siblings, stating, "The fundamental right of adult siblings to sexual self-determination has more weight in such cases than the abstract protection of the family."

"What [these feminist activists] did, essentially, was move the law from this moralistic account, which often treated women as potentially seducing fathers, brothers, uncles, etc., and focused more on a clinical language of sexual violence, culled from psychological studies and survivor testimonies," explains Brian Connolly, author of Domestic Intimacies: Incest and the Liberal Subject in Nineteenth-Century America . "So incest became attached to rape law rather than family law."

"If I can consent to have sex with an entire football team, why can't I consent to have sex with my brother who is over the age of 30?" she adds, her voice rising. "It just doesn't make sense to me that I can consent to one but not to the other. I don't think that is correct at all."

The increased risk of having offspring with certain genetic disorders is another legitimate concern in incestuous couples: According to Psychology Today, there is an "astonishingly high" chance that offspring born to close relatives will be born with a serious birth defect. However, those who advocate for decriminalizing sibling incest maintain that this risk does not justify anti-incest legislation.

"Even if siblings have a higher risk of having a child with a severe disability [or] disorder, we should not punish people who have a higher risk of having a disabled child," says Petersen. In addition, he notes, "I have heard that you can have sex without having children."

But even if you remove all the typical legal objections to incest, it still invokes a sense of strong moral repugnance in a majority of people. In a 2000 study, for example, social psychologist Jonathan Haidt presented research subjects with a fabricated incest scenario that he had intentionally written to be "simultaneously harmless yet disgusting":

Julie and Mark, who are brother and sister, are traveling together in France. They are both on summer vacation from college. One night they are staying alone in a cabin near the beach. They decide that it would be interesting and fun if they tried making love. At very least it would be a new experience for each of them. Julie was already taking birth control pills, but Mark uses a condom too, just to be safe. They both enjoy it, but they decide not to do it again. They keep that night as a special secret between them, which makes them feel even closer to each other. So what do you think about this? Was it wrong for them to have sex?

Eighty percent of subjects polled said that Julie and Mark were morally wrong to have slept together. When pressed to explain why, "participants often directly stated that they were dumbfounded" and that "they thought the action was wrong but could not find the words to explain themselves."

Even within the online GSA community, there's dissent regarding whether those who struggle with feelings of genetic sexual attraction should act on their desires. On the GSA Forums, a private forum with nearly 300 members and 11,000 posts, many of the most active posters constantly urge others to refrain from "crossing the line," as they call it, even though most agree that anti-incest laws are discriminatory.