Consanguinamory Happens, and Sometimes Children Are a Result

Via Full Marriage Equality:

A History Of Incest

Incest is defined as having sexual relations with close family members, and throughout the majority of the world the practice is not only taboo but also illegal. [Yes, but it's a slim majority.] However, just because people won’t admit to partaking in incest does not mean that the practice does not occur. In some circumstances, incest is a necessity  and multiple studies have shown that offspring of distant relatives are actually healthier than the general population.

There is usually more of an evolutionary advantage to diversifying the gene pool, [wrong, it's diversifying the genes of your offspring that counts,] and for this reason incest in not very popular in species who practice sexual reproduction. [That depends as well.] However, in species where there is no natural advantage to genetic diversity, incest still exists. For example, Dr. Nathaniel Wheelwright, an evolutionary biologist at Bowdoin College in Maine who focuses on sexual reproduction, described asexual reproduction to LiveScience  as “the ultimate incest” because an organism is breeding with itself. "You can still see species asexually reproducing, or cloning themselves, in situations where there is no advantage to [sex]," Wheelwright explained, "and you can see species that commit incest where there is no penalty to inbreeding."

Kissing Cousins

[...] When you have an even smaller gene pool, such as two first relatives, the chances of inheriting these recessive conditions may skyrocket to 50/50. Interestingly though, this is not the case for all inherited disease, as many need generations of inbreeding before they can ever be expressed. A 2008 study on 48 cases of incest found that the risk for birth defects is around two percent in the general population but rises to only four percent between first cousins. However, due to the sensitivity of the subject, there was no information available for the children of incest between closer relations.

What Draws Some To Incest

It’s believed that we have a biological defense against close forms of incest, since these are the cases most likely to end in genetically compromised offspring. LiveScience reported that Finnish sociologist Edward Westermarck suggested that growing up in the same house puts people off from developing sexual feelings. This remains even in cases where children are not directly related. When close incest does occur, that is, sexual relations between first relations such as brother and sister or father and daughter, it is more likely due to a psychological phenomenon than a biological attraction. Genetic attraction occurs when two relatives who have been separated for the majority of their life meet for the first time and experience an intense emotional attraction. As reported by The Guardian, it occurs in around 50 percent of reunions between close relatives separated at birth.

[...] It’s when these individuals act on those feeling that GSA becomes incest. Soll explained that this jump to incest is most common in brother-sister relationships, although it’s unknown why. According to one brother and sister relationship, the intense attraction is rooted in their physical resemblance. "It's like kissing myself," a woman who goes by the name "Rachel" told ABC News of her intimacy with her brother "Shawn."

It's known that animals, including humans, have various sexual impulses. Some urge us to seek out more genetically similarity (and thus more closely related) to us, while others urge us to seek genetic novelty. In the wild, usually a balance between the two is optimal. It's also not unheard of for people to experience this in inverse: when they look in the mirror, they see their lover in themselves, a sort of inverse of narcissism. I'd also like to point out that plenty of couples, including GSA couples, do not look similar at all.

The point is, it's complicated.