Midwives have been told about the benefits of “close relative marriage” in training documents that minimise the risks to couples’ children.
The documents claim “85 to 90 per cent of cousin couples do not have affected children” and warn staff that “close relative marriage is often stigmatised in England”, adding claims that “the associated genetic risks have been exaggerated”.
Should disabled people be blanket banned from having sex or children? No obviously not. Not really workable anyways and quite morally hazardous to put into law, as you point out.
Should people with disabilities ought to (in a moral sense) have children that are at high risk of sharing their disability? Also no. To be frank, there’s a reason we call it disability. Even though they can have good, rich, valuable lives, they much more often don’t.
This is definitely a question of degrees. Society and medical support can change this line. Like where diabetes used to be a death sentence now it’s serious but treatable. So less problematic to pass on diabetes today vs 200y ago, but why would you want to?
Finally let’s get to cousins. Beyond the additional risk that they have children with health problems, there’s a question of consent. Even between cousins (like siblings) there’s often a power dynamic that makes consent hazardous. So IMO, obviously immoral. Making this illegal is not very restrictive (it affects you banging like 0-100 specific people out of literally billions) and codifies what was a taboo anyways (which is like, a pretty significant amount of law). 24 US states agree with me.