Those that say no, are usually ignorant to the damage they do to future generations and their kid that could possibly be born with a debilitating disease. They are the cause of so much stress, sadness, and depression, but they don't necessarily care, what they do care for is their relationship and for the experience of having a kid.
Think about it. Incest is banned because of an increased risk of genetic disease. Two people with the same exact recessive genetic illness will 100% of the time have kids who also have that same illness. So since incest is illegal because of a much lower risk it follows that to be consistent we must make it illegal for two people with the same recessive genetic illness to marry or even to have sex.
It is completely different from incest. Unless a person gets genetically tested, she is unaware that she is carrying a receive allele for a genetic disorder. It would be smart for a couple before they conceive to get a genetic karyotype so that way the couple knows if and when they should have children.
Unless they are related it should not be illegal. I have a son with the thalassemia trait and he should net be punished for this. It is a genetic disorder, it did not come from incest. If he has a child with the same trait, they have a 25% chance of their child having the disease, and a fifty percent chance of having a child with the trait. Since we have knowledge of this disorder, we can do something about it. Education is the key.
It should not be illegal for people with the same recessive genetic illness to marry or have sex. This is because it would be highly subjective to what constitutes a genetic illness. Also, many people do not know they have a recessive genetic illness, so how could you charge them with a crime when they find out, it wouldn't be fair.
That should not be legal because they will give life to any child and there's a 99 percent chance that the child will be born with that kind of genetic illness, and the world will be more and more overpopulated with sick people. That's why this should not happen so if anybody is in position to stop this, stop it as fast as you all can.