Half the states in the union permit marriage between first cousins. Children of such couplings are at a 2-3% higher risk of birth defects and genetic anamolies than the progeny of couples who are not so closely related to one another. However, genetic and family counseling can greatly reduce this risk, and most physicians agree that, while non-trivial, the danger is eminently manageable.
By contrast, only six states allow matrimony between members of the same sex. These unions carry no additional health risks for children in families with homosexual parents, and have been shown to place them at no disadvantage socially or otherwise.
It would appear then, that in matters of marriage, 26 trust states adults to make informed decisions about risks to their offspring when the parents or potential parents are related to one another. In 44 states, however, parents whose matrimony creates no additional risk for their offspring are simply denied the right to marry for no other reason besides prejudice.
Or am I missing something?