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."
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.
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."