That might be true if they were evenly distributed across the world, but as they'll be clustered rather tightly near the donor, the chances of them unknowingly

That might be true if they were evenly distributed across the world, but as they'll be clustered rather tightly near the donor, the chances of them unknowingly having kids together goes up quite a bit. Part of our aversion to that is probably just cultural, but genetics could be a problem.