I think the two issues are connected, simply because both are instances of political interference in the personal life choices of the responsible individual. Yes, marriage between cousins carries some risks of genetic disease. But so does marriage between persons of African ancestry - sickle cell anaemia, to be exact; and between persons of mediterranean ancestry, in the form of thalassaemia. And no state would dare ban marriage between persons of those ethnic groups on the basis of the potential medical consequences for the offspring. I believe marriage (if you really feel the need of a formal document or ceremony and you live in a country where that is required for full recognition of your partnership) should be allowed between any consenting adults (18+, that is; under that age, while you may want sex and understand the consquences, you should not be allowed for your own sake to sign to a formal commitment of that kind) who are compos mentis and not direct kin like siblings or parents. The genetic issues should be left to blood testing and medical advice, not to proponents of doctrine.