[weegee]

Blood related couples are prohibited from marrying because the offspring they produce could be prone to genetic defects.

Either the government can regulate marriage or it can't.

[Luis Gonzalez]

Your rights end where mine begin.

The right of blood relations to marry end when the rights of the unrealized issue of their union come into play.

By the way, a great number of States OK the marriage of blood relatives as long as there is an agreement, or no possibility, of any issue.