"When the curtains close, my brother and I go from siblings to lovers."

Katherine* is in love with a man called Scott*.

Scott is head over heels for the woman he first met at an airport.

The only problem: Katherine and Scott are half-siblings and where they live in America, incest is illegal and akin to child sex abuse.

The two have been dating for three years, after first stumbling upon one another on the internet.

Speaking to Broadly, Katherine says that she was adopted at a young age and at around 18 she started looking for her biological family.

Katherine was adopted at a young age.

Despite making some connections with family members, no connections with them were long lasting.

At 32, Katherine decided to get a Facebook account and used the last name of her late biological father.

Scott, who quickly identified Katherine by her last name, friend-requested and from there they started messaging.

"[We had the] same interests, similar thought processes," Katherine said. "We liked a lot of the same things: favourite colour, stuff we liked to eat - just, you know, general things."

But, very quickly their relationship deepened after they shared photos of each other.

"When we swapped pictures, it was like a ton of bricks hit me. It was like looking at myself in a male version. I was attracted to him immediately, but I didn't know if he was attracted to me or not."

They also discovered similar interests broader than favourite colours - BDSM. Katherine linked Scott to her images from her profile on a fetish site, that weren't X-rated, but definitely sexually attractive.

Scott then confessed to Katherine he'd been "having thoughts" about her.

Initially, the pair were confused by their situation but after Googling it, they found Katherine and Scott had genetic sexual attraction (GSA).

Don't know what GSA is? Here's an explainer. (Post continues after video.)

Learning that she wasn't the only one in the world, Katherine felt far more comfortable with the situation and decided to pursue her relationship with Scott.

They first met at the airport, when she flew across the country to meet Scott face to face.

"We were grinning like kids in a candy store. 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."

Driving home, they pulled over and kissed one another for the first time.

Katherine says that her connection with Scott as a sibling makes their love closer than most.

"In some instances, being related actually makes you a bit closer. 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."

Scroll through to see some on-screen couples who were related. (Post continues after gallery.)

Despite dating for three years and having moved in with one another, the two haven't told a soul. Their family members think they are just living together as siblings and when they're home alone, they close the blinds.

Katherine thinks if the pair were found out, they could both be jailed for 15 years.

"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. "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 has now become an advocate in the GSA community, writing on her blog, Lilys Gardener.

She writes, "[You should have] the right to be with whomever you choose as long as all are consenting adults should be a basic human right."

With incest being compared to child sex abuse, Katherine avidly opposes such a view.

"People link incest to other sorts of things, such as abuse and pedophilia 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.

"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 next question is, will Katherine and Scott ever come out to their families?

Well, only if they move to what they consider "safe states", where incest is not directly criminilsed.

Once living somewhere like New Jersey, Ohio or Rhode Island, the couple won't be hiding anymore.

"If we move to a safe state, we're throwing a big party, and we're letting everybody know what we have between us and how good we are together and how much we love each other.

"Once we can be open, I would like to do something to change the laws so that we can get married one day. You know, laws change every year... Just because it's the law doesn't mean it's wrong."

*Names have been changed to protect the privacy of the individuals.