When the gay marriage debate recently erupted a lot of religious people made arguments that extending the definition of marriage to gay people would devalue the institution of marriage as well. Even Chief Justice Scalia weighed in suggesting that the institution of marriage is not a government construct, it preceded the establishment of the government and it is not something that the government really has the authority to define.
So now here is a psychologist who is weighing in. I have heard him on different issues and disagreed with him. But, he raises some interesting points – one of them being that historically the state has had an interest in the greater society providing children in households with a mother and father.
Read at your own risk.