There are a few things to understand, here.
Islam does not require cousin marriage, but it doesn’t prohibit it, either, and as far as I’m aware, the practice predates the religion.
Thresholds matter for social behavior. Something that is okay in small amounts may not be okay in larger amounts.
Here is a map of 2nd-degree-or-closer cousin marriages from Wikipedia.
Notice what a deep blue Pakistan is. Also notice that in most Western countries, the level is fairly low.
Now let’s hop over to the Biological Aspects section of the Wikipedia page.
In April 2002, the Journal of Genetic Counseling released a report which estimated the average risk of birth defects in a child born of first cousins at 1.1–2.0 percentage points over an average base risk for non-cousin couples of 3%, or about the same as that of any woman over age 40.
Well now, that doesn’t sound so dangerous - wait, what’s this following paragraph?
Repeated consanguineous marriages within a group are more problematic. After repeated generations of cousin marriage the actual genetic relationship between two people is closer than the most immediate relationship would suggest. In Pakistan, where there has been cousin marriage for generations and the current rate may exceed 50%, one study estimated infant mortality at 12.7 percent for married double first cousins, 7.9 percent for first cousins, 9.2 percent for first cousins once removed/double second cousins, 6.9 percent for second cousins, and 5.1 percent among nonconsanguineous progeny. Among double first cousin progeny, 41.2 percent of prereproductive deaths were associated with the expression of detrimental recessive genes, with equivalent values of 26.0, 14.9, and 8.1 percent for first cousins, first cousins once removed/double second cousins, and second cousins respectively.
Oh dear.
A BBC report discussed Pakistanis in Britain, 55% of whom marry a first cousin.
Oh no. No no no.
Given the high rate of such marriages, many children come from repeat generations of first-cousin marriages. The report states that these children are 13 times more likely than the general population to produce children with genetic disorders, and one in ten children of first-cousin marriages in Birmingham either dies in infancy or develops a serious disability. The BBC also states that Pakistani-Britons, who account for some 3% of all births in the UK, produce “just under a third” of all British children with genetic illnesses. Published studies show that mean perinatal mortality in the Pakistani community of 15.7 per thousand significantly exceeds that in the indigenous population and all other ethnic groups in Britain. Congenital anomalies account for 41 percent of all British Pakistani infant deaths.
Well, fuck. This isn’t good.
The increased mortality and birth defects observed among British Pakistanis may, however, have another source besides current consanguinity.
Oh, you mean it might be some kind of outside oppression? I bet Whi-
Population subdivision results from decreased gene flow among different groups in a population. Because members of Pakistani biradari have married only inside these groups for generations, offspring have higher average homozygosity even for couples with no known genetic relationship.
Oh. Nope, having kids with people who are too genetically similar to each other.
Now remember, we’re talking about information from Wikipedia and the BBC, not Evil Hatefacts from an Evil Hatesite.
So that’s the genetic aspect. So why do they do it?
To keep wealth within the family and stick close to the father’s genetic line.
It isn’t some huge, secret magical diverse cultural benefit that the Middle East has and we don’t. It’s just clannishness. (In fact, I suspect the clannishness is even responsible for some of the issues in their armies.)
Now, the Alt Right seems to think that as a result of this and other issues, all Muslim immigrants must be kicked out of the UK.
That is not necessary. Also it would probably get a lot of people hurt or killed, which is bad. So let’s not do that.
For the Liberals, we should keep in mind that cousin marriage likely promotes clannishness and amoral familism (”my family, right or wrong”), due to increased genetic similarity and insulation from the outer world. In fact, that’s pretty much the purpose of the practice.
Remember that social atomization that was supposed to melt away the religions and make everyone into happy Liberals? That isn’t going to happen if they all marry their cousins, which enables and incentivizes close, repressive, tight control of women, and insular culture.
If we want Islam to chill out and liberalize and soften, like Christianity, and we want the Muslim immigrants to become happy Liberals, then we must ban cousin marriage.
No excuses because “it’s their culture,” or “you’re just a repressive [ethnic majority].” That isn’t helping them.
That’s enabling them. Cousin marriage is bad. It’s self-destructive behavior. It’s other-destructive behavior for the kids, too.
Human beings are resilient. It won’t take that long to start removing the most negative effects, if we start now.
You went straight from “this is a problem” to “we must make this illegal” without any consideration of non-violent solutions.
Considering the current level of ongoing cousin marriage, the current “compromise” clearly is not enough. It is better to have a new “compromise” which is substantially more opposed to cousin marriage than the current one.
So tell me, how do you stop the Progressive Left from decrying the new Reproductive Genetic Risk Assessment Tax as Evil Racism? And how do you prevent it from expanding to other categories?
“Don’t marry your cousin, the government will no longer issue any new marriage licenses for it after [DATE]” is a pretty clear line that draws on the pre-existing taboo in the West against incest and is therefore unlikely to dramatically expand, or cost much to administer.