Feb. 19, 2007  -- Rachel and Shawn's love story sounds like something out of a fairy tale. They say it was love at first sight when they met almost eight years ago, and they have lived together as a happy couple ever since. But there is one crucial difference that sets Rachel and Shawn apart. They are part of a phenomenon known as genetic sexual attraction, or GSA.

Psychotherapist Joe Soll says that, "genetic attraction is an attraction between two people who've been separated ... seeing someone they've missed all their life, all the emotion of that loss, sometimes turns into a sexual relationship." You see, Rachel and Shawn are half brother and sister.

Rachel and Shawn -- who didn't want their real names or where they lived revealed -- were born only 28 days apart, from mothers who were pregnant by the same man. Despite having the same father, the two never met until they sought each other out at the age of 27. Rachel believes growing up separately is key to understanding their love.

"Had we grown up together as siblings, as children," Rachel says, "this would have not existed."

Regardless, they know they are breaking a social taboo, and they say they understand why people might call it incestuous.

"We are related," Rachel says. "We're not going to deny that."

'It's Like Kissing Myself'

Rachel and Shawn insist they are just like any other couple.

"I'm a normal guy," Shawn says. "I'm in fantasy football, I fish, I do everything that they do. I'm a normal person."

Rachel says they go to work, eat dinner, watch TV and go to sleep just like other couples. And just like other couples, they have an intimate relationship as well. But unlike everyone else, they believe their attraction is actually heightened by their genetic similarities.

"It's like kissing myself," Rachel says.

After four years together, Shawn decided to propose to Rachel. However, they are not married, because no state in the country allows brothers and sisters to legally wed. Rachel does not want to have children, but the couple says they would not be afraid to start a family if she did.

"I have two different sets of friends that are together," Rachel says, referring to other brother-sister couples. "Their children are perfectly normal."

But is it safe for half brothers and sisters to procreate? No studies have been conducted on the offspring of siblings, but social anthropologist Martin Ottenheimer has done extensive research on first cousins.

"The myth is that cousin marriage, or close inbreeding, produces stupid, deformed people," Ottenheimer says. "But that's not the case."

A report from the Journal of Genetic Counseling indicates that cousin couples have only 2 percent more of a chance of having children with birth defects as compared to unrelated couples.

Going Back to the Bible?

Ottenheimer also points out that sibling marriages were not always such an aberration.

"Brothers and sisters married in a number of different societies," he says. "This happened in Peru. This happened in Egypt. This happened in Hawaii."

But while Ottenheimer believes it is time to change the incest taboo against cousins, he doesn't think we need to go as far as accepting brother-sister couples.

"Does that mean that we have to allow brothers and sisters?" he asks. "No, it doesn't mean that. Nor do we have to allow parents and children to legally have sexual intercourse. We don't have to allow it."

Rachel and Shawn are devoutly religious, and even point to the Bible as a precedent for their love.

"If you take Adam and Eve," Shawn says, "where did we all come from if there wasn't incest?"

Shifting Emotions

Genetic sexual attraction is not a phenomenon that is exclusive to sibling couples. According to Barbara Gonyo, a 70-year-old grandmother in the Midwest, it can also occur between parents and their estranged adult children.

Gonyo was only 15 years old when she discovered she was pregnant in the early 1950s. Her parents forced her to give up her baby for adoption, and she remembers how traumatic it was to watch the nurse at the hospital immediately take her baby son away from her.

"'Say goodbye to your mommy now'… was all she said," Gonyo says. "It's like somebody put a rock in my heart. It was horrible."

As the years passed, Gonyo never stopped wondering about the son she lost. She sought him out, and almost 30 years ago, when he was 26, they were reunited. The first thing Barbara noticed was how much her son looked like his father, the first love of her life.

Gonyo's son made it clear to her that he did not want to call her "mom" out of respect for his adoptive mother. As Barbara realized she couldn't be his mother, her emotions began to shift.

"If I can't be his mother, who can I be?" she asked herself. "So I started to feel more romantically towards him that I did motherly."

Gonyo was sickened by her feelings and feared she was crazy. But she soon began to understand her feelings to be genetic sexual attraction. She believed she had feelings of attraction because she had missed out on bonding with her son.

Joe Soll, who wrote a book called "Adoption Healing," says GSA often happens in people separated by adoption.

"They want to be close and hug because they haven't had the relationship for 20 or 30 years," he says. "And that hug can turn into something else."

'If We Act on It, It's Called Incest'

Not everyone believes GSA is a legitimate phenomenon. Adoption expert Adam Pertman says genetic sexual attraction is no more common a phenomenon than incest.

"It happens in adoption just like it happens in biologically formed families," he says. "It's rare in adoption just like it's rare in biologically formed families. If we act on it, that's called incest, and we don't think that's a great idea for lots of good reasons."

Gonyo agrees that acting on those feelings would have been a horrible mistake. Even though she had an intense attraction toward her son for 15 years, her son never reciprocated, and she never acted on desires.

"Emotionally," she says, "you are the adult, and they are the child. If you go through with the sexual act, then I hold the parent responsible, not the child."

But half siblings Rachel and Shawn don't feel the need for restraint. They couldn't be happier that they finally met and fell in love.

"I apologize to everybody out there that's a nonsibling couple, but you will never have what we have ever, ever, ever," says Rachel. "It just simply cannot exist outside of what we have."