It's like many forms of incest, there's typically a power dynamic that results in a victim. Not to mention, society pays the price of covering children born at a higher risk of genetic disorders. Morally, there's nothing wrong with two consenting adults getting it on, practically, it's rarely consensual in incestuous relationships.

He's talking about marriage

Seriously. It takes generations for bad things to happen to the genes. You could ban them from having children together and that would theoretically solve the issue.

I just don't see why people are acting like this is a massive issue.