I've got about half-way through this. There's a couple of things I want to put my oar in over:
- eugenics, specifically addressing points made by Mooncup
- arranged marriage as a form of human trafficking
Eugenics
The bad thing about historic eugenic practices isn't that it made fewer disabled people be born, but that the human rights of would-be parents were violated. I'll explain why:
- An unconceived person has no human rights because they don't yet exist. We don't have to even consider their future human rights until someone starts TTC.
- A would-be parent absolutely has human rights that should be respected, including the right to marry and found a family.
- With the exception of the right to freedom from torture, no human rights are absolute but can be limited for reasons of the public good. These reasons can include public health.
Ripping out people's reproductive organs, banning inter-racial marriage, etc doesn't come close to qualifying as legitimate limitation of the right to marry and found a family, because the former stops someone from having kids at all and the latter stops someone from marrying hundreds of thousands or more people based on skin colour or national origin. Additionally, forced sterilisation is arguably a form of torture.
Likewise, bundling disabled people into gas vans to murder them is a clear breach of their human right to life.
So all that stuff that happened in the US in the early 20th century, and later on in Germany, is absolutely violating people's rights: the rights of born people. No unconceived people's rights were violated because no one can have rights prior to conception. (Whether forced abortion violates the foetus's rights depends on your views of the concept of foetal personhood and settling that debate is way beyond the scope of this post.)
Stopping someone from marrying a specific small group of people based on proximity of kinship isn't an unreasonable limitation on the rights to marry and found a family because the people can still have kids and they can marry any of literally millions of people. Adding first cousins to the list of people you can't marry is a warranted and very small limitation on the right to marry and found a family, given the harms caused by repeated cousin marriages demonstrated by Born In Bradford and other research.
Stopping someone who is likely to have a congenital genetic disability from even being conceived by preventing two cousins from marrying and founding a family together isn't a breach of that non-existant person's human rights because they aren't a person to even have rights yet. To the contrary, we have a moral imperative to ethically attempt to minimise the likelihood that someone is born disabled because of the adverse impact that disability will have on that person's life, in just the same way that we don't let pregnant women take thalidomide even if they are on a drip for hyperemesis gravidarum.
Arguably, because some people believe in foetal personhood, preventing a conception by preventing a marriage and consequent sexual union is more ethical than IVF and discarding embryoes that have genetic problems, and more ethical than in-utero testing followed by abortion.
Lastly, it's less discriminatory against disabled people to have a blanket ban on first cousin marriages than it is to have a case-by-case assessment based on gene testing, because gene testing will result in more disabled people than non-disabled being told "no, you can't your cousin, even though your brother can". Example: cystic fibrosis, recessive genetic condition, someone with it has two copies of the gene so if they marry a cousin with one copy, half the kids will have CF and half will be carriers, so they get told "no", but their brother who lucked out and isn't even a carrier is told "yes".
Banning first cousin marriage isn't a slippery slope to forced abortion etc because it's a completely different, purely legislative, intervention that violates no one's rights.
Human trafficking
In the half of the thread I've read so far, no one seems to really consider arranged marriage as a form of human trafficking. You've got marriages that the couple can't really say "no" to and these marriages are absolutely being used to get British-born people's overseas cousins into the UK on spousal visas. And when it's a woman brought over on a spousal visa, her ability to flee DV is limited greatly by her not being British, possibly speaking little English, and having "no recourse to public funds" stamped on her visa.
We know from Jewish women that an abusive man will look overseas for his arranged marriage bride to overcome the refusal of every family in his local community to let their daughters wed him, because families get reputations for their men being batterers and the parents in the community learn to avoid them when picking matches for their daughters. What makes anyone think that abusive men in other ethno-religious communities won't pull this stunt to get wed?
If we demanded proof that marriages performed overseas were not first cousin marriages and refused to recognise first cousin marriages for immigration purposes, we could reduce this form of trafficking and consequent abuse, because a) the parents of the UK-born spouse involved will be less motivated to get a more-distant relative into the UK and b) being married to a second cousin instead of a first cousin means fewer relatives in common, more relatives not in common, and so more people who might take a DV victim's side if she opts to leave.