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”.
The issue I have with your argument is you can use the exact same argument for sibling incest. If two cousins can have a child, and we’re dismissing the birth defect risk argument, then why can’t a brother and sister have a child? What if they just want to fuck? What if the entire family is into the aristocrats style gang bang?
Your argument doesn’t draw a line between cousin incest and parent-child or sibling incest. If one is okay then the other should also be okay and I don’t know about you but I’m definitely not okay with the latter. I’m not saying you’re in the wrong but I do disagree with the argument you made for it.
Parent-child incest has the power dynamic issue. It’s basically impossible to consent in that relationship. As to siblings, I’d argue that the logical conclusion is that it is probably okay, unless there’s a limit to how much birth defect risk is allowable, which as noted above, comes with other issues.
Siblings definitely have power dynamics that make consent very hazardous. I’d argue first cousins also have such dynamics. Perhaps to a lesser degree, but there’s no real benefit from having cousins marry and there is an increased risk of birth defects, so better to disallow it.