Why should it be illegal to have a relationship with someone you’re only related to by law?
Logically only the same reason you couldn’t have a relationship with first cousins. Inbreeding isn’t exactly a problem for first cousins, they’re genetically different enough for it to not have much of an affect until multiple generations of it (plus same-sex people, sterile people, people who just won’t have kids), so the only plausible argument for it is “marriages between family members are more likely to be from grooming/manipulation/abuse”. Which I don’t think is flawless reasoning to make it illegal, same thing could be said about many other perfectly legal types of relationships. But it is a reason.
Logically only the same reason you couldn’t have a relationship with first cousins. Inbreeding isn’t exactly a problem for first cousins, they’re genetically different enough for it to not have much of an affect until multiple generations of it (plus same-sex people, sterile people, people who just won’t have kids), so the only plausible argument for it is “marriages between family members are more likely to be from grooming/manipulation/abuse”. Which I don’t think is flawless reasoning to make it illegal, same thing could be said about many other perfectly legal types of relationships. But it is a reason.