I don't care as long as they are consenting adults.

When it comes to reproduction, it's probably for the best that both family history and both partners are examined for genetic disorders. Even for unmanifested disorders, If they both have some defect on a recessive gene, the kid's chance of having the disorder are way higher (especially if a boy).
Still, I don't see much point in making it illegal since it's not the only thing that can increase that risk. Older (completely unrelated) partners could already have some errors in the DNA replication (though in the case of cross-generational incest that's likely to already be an added weight), Down's syndrome occurances jump pretty high as the maternal age goes up, alcohol/smoking/other drugs during pregnancy can also cause such problems.