We are not talking about anything involving coercion or force or molestation. There are laws against rape, assault, and molestation, and they should remain. We are talking about consensual incest, consanguineous sex and marriage, and consanguinamory, whether initiated through Genetic Sexual Attraction or not.
Short answer: It isn’t illegal everywhere, but where it is, it is the lingering result of sex-police holdovers, superstition, prejudice, and legislative inertia.
A significant part of the reason is that some cultures have an ancient taboo against incest, which begs the question, “Why is incest taboo?”
There seems to be more than one plausible reason why some cultures have had an incest taboo, in addition to protecting the power of the leaders.
1) The taboo appears to be an adoption as law or culture of the biological Westermarck effect, which is a common experience but not one experienced by everyone. Because of this effect, many people develop an avoidance of their close family members as sexual partners, and people have often expected that everyone feels the same way they do, and in many cases try to discourage people from acting differently than they do. But for those who, due to separation or some other reason, don’t experience the Westermarck effect, the bond becomes especially strong if it grows into a consanguinamorous one. It could be that if nobody had experienced the Westermarck effect, most people wouldn’t have ever “left the nest” (unless driven out by the dominant male) and the human race would have had a harder time developing genetic diversity, which was very important to survival when the human population was low. Without a growing and genetically diverse population, the entire population could have easily been wiped out by one disease. We hardly have that problem anymore. So in that respect, the Westermarck effect is vestigial and the taboo is no longer needed. (Note that the need for a growing population was also important when everything was accomplished through much physical labor, requiring many people. This was also one of the reasons why some cultures discouraged same-gender pairing that excluded males from bonding with females and making babies.)
2) Also, in patriarchal societies, it was common to trade daughters away in a business deal or to form alliances with other clans or nations. Especially if virginity was valued in new brides, it didn’t help matters if she was in love with, and making love with, her brother. But as with the previous reason, life has changed much and fewer segments of humanity are trading daughters like bargaining chips.
3) When the entire town or village was expected to attend the same church or temple, the taboo was reinforced if that religion had a prohibition against it. But in many places, this is no longer the case.
Incest was one of many things prohibited in the ancient nation of Israel, per the Torah. Church and political authorities have found incest prohibitions useful not only as part of overall control of sexuality, but for making accusations against opponents and the inconvenient (and how does one prove that they didn’t have sex with someone else?)… and to prevent any one other family from building up and retaining too much power. While royals in Egypt, Hawaii, Europe, and elsewhere married siblings, cousins, and other relatives to retain power, they often denied other people that right for the same reason.
The religion-imposed taboo should not be underestimated, and leads us into the Other Reasons Incest is Illegal in Some Places.
In many places, there has been an official national or state religion, perhaps with a religious organization having at least some direct influence on the laws. Even in the US, where the Constitution now guarantees freedom of religion and there has been a firmly established separation of church and state, some states were originally colonies established with their own official churches. New states often set up their laws by copying from existing states, and the US has had some Puritan origins, later Victorian influence, and so forth. Famously, alcohol was banned under Prohibition less than a hundred years ago. Before that, women couldn’t even vote and there were many restrictions placed on women that were not placed on men, and thus there was gender inequality under the law and a woman was more or less the property of her father or brother until or unless she married, at which time she became the property of her husband.
Female schoolteachers were expected to abstain from sex or resign if they married. Female pageant contestants had to swear that they were virgins. Until recently, it was common for college dormitory buildings (if not the college itself) to be segregated by gender, complete with curfews and supervisors to try to make sure that students, who were 18 years and older, were not having sex (heterosexual, anyway). Innkeepers, landlords, and property sellers would routinely (often by law) refuse to accommodate or do business with unmarried, mixed-gendered couples.
Boys and young men routinely faced criminal charges for consensual sex with females (or men of any age could be civilly charged with “breach of promise”.)
It was common to have laws against anything but heterosexual intercourse between a husband and a wife. That meant oral sex between a husband and wife was technically illegal, as was any gay or lesbian sex (gay bars were raided by police), unmarried sex or cohabitation; even sex toys and birth control have been illegal in some places. There are still places in the US where someone can be sued for “alienation of affection” for having sex with a married person. Never mind that, even where illegal, brothels have always existed, and fathers have taken their sons to them for their son to have pleasant sexual encounter with a professional, and have mingled with people in power as fellow customers. Never mind that quietly having lovers on the side has been something that has always taken place.
There have also been, and in many ways remain, laws against and restrictions on various forms of dancing, nudity, “crossdressing,” and erotica.
With this sex-negative attitude, it isn’t surprising that there have been laws against incest. What may be surprising is that such laws have remained on the books. There has been a progression of civil rights in places like the US that is moving towards an adult having the right to share love, sex, residence, and marriage with any consenting adults, but we’re still not there. Loving v. Virginia struck down bans on (heterosexual, monogamous) interracial marriages. The Lawrence v. Texas decision struck down laws against gay sex and in 2015 the limited monogamous same-sex freedom to marry became nationwide in the US. Cases regarding polyamory are currently winding their way through US courts.
In order for remaining laws against consensual incest to be removed, we'll likely need a good test case for the courts. What that would require is a respectable and otherwise law-abiding (and attractive wouldn’t hurt) long-term consanguinamorous couple to fight a state law against consensual incest. The catch-22 is that since people can be, and are, prosecuted for engaging in this consensual relationship, lovers have a strong motivation to hide these relationships, and that is a hindrance to getting the laws changed. It would help if a couple in a state, such as Rhode Island, with no law against consensual incest, applied for a marriage license and subsequently got the courts to overturn prohibitions on consanguineous marriages. However, in addition to fear of prosecution and other legal problems, some people who are, or have been, involved in consanguinamory would prefer the law and/or the taboo remain in place, either because they like being the rebel, they are self-loathing, or they can’t (anymore) have what they want and they don’t want anyone else to have it, either. But they are the minority; most people involved in these relationships very much want their rights.
Throughout all of history, around the world, royal or peasants, urban or royal, rich or poor, there have been close relatives engaging in experimentation or having lifelong spousal-type relationships, and everything in between. You know people who have been involved in consanguinamory whether you know it or not, and whether or not your genealogical charts reveal it, chances are that you don’t have to go too far back in your family tree to find an ancestor whose true biological parents were close relatives.
There are people in relationships right now who would benefit if they had their right to marry. There is now no good reason to keep these laws and the taboo that deny an adult the right to share love, sex, residence, and marriage with any consenting adults. We’re not all going to want the same love lives as each other, but we should allow people to have the relationships of their mutual choosing, the ones in which they will function best.
An adult should be free to share love, sex, residence, and marriage with any and all consenting adults without being subjected to prosecution, bullying, or discrimination.
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