If you look at "royal intermarriage", that did occur all up towards ww1. The "dangers" of inbreeding didn't start to get really known until the study of genetics arrived in the the early twentieth century.
Yet they were fully theorized by Darwin in 1839, therefore before (and outside of) the publication of his Theory of species. And you don't need to look at royalty to find official incestuous couple up to the end of the 19 century, starting by Darwin.
It's not like two "healthy" family members with no problems, start make two headed babies if they jump in the sack. But if two people who carry the same recessive genes get an offspring, the recessive genes would get reinforced. Now carry on that for some generations and you will get issues for sure. But, a one off with healthy beings? Doubt it would churn out a hellspawn.
Even with unhealthy beings it wouldn't churn out a hellspawn. The studies on Neanderthal's genome proves that they were deeply inbreed. It surely prevented them to evolve more and contributed to the limitation of their population, but it didn't led to the end of the specie, which slowly blended with Homo Sapiens.
Another proof of that is the
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, one of the few last uncontacted tribes on Earth, and the more secluded one. They live on a small island since who know how many millennium, and are more than surely deeply inbreed.
It's more a religious and cultural thing.
At the level of an individual, yes, surely. But at the level of the society, it's more a question of well being of the possible offsprings ; both because of the risk for their health, and the way others could react to who are their parents. While the said parents are consenting adults, the offspring had no say on this and would be, in the end, the only one to effectively suffer from the situation.
Which led the, many, countries where incest is legal, to limit this legality and either permit it between 1st cousins or lower bound, or making it illegal for such couple to have children.
You don't justify bribery by saying it's okay because it's between two consenting adults. You have to take consequences into the factor too.
Oh, an eugenicist, long time not seen one.
Incest also pollutes the gene-pool of the country.
More surely than with none inbreed child, but not really more often than them. Direct relatives aren't the only ones to share recessive alleles, they just are the ones for who have more risk to share them. The effective risk don't come from the inbreeding, but from its repetition, generation after generation.
You might argue, they can take precautions, but you know it doesn't happen always, then you end up seeing so many innocent children with genetic defects.
Oh yeah... so many... there's 5.21% of the children that have birth defect in countries where consanguinity is an exception, against 6.99% in countries where it happen (relatively speaking) often ; yet in two of those countries, the rate is lower than 1.5% [
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].
Of course, the said rate is subject to caution, since it depend of the locale definition of "birth defect". Yet even with a large interpretation it just pass the risk from 5% to 10%.
So many children are suffering because of reckless teenagers, and this will make it even worst. So just don't involve children (directly or indirectly) into fetishes.
I'm pretty sure that incestuous couples are in a relationship because of the love they feel for each other, and not because of a fetish they are sharing...
