The linked post from statenews.com is a very good read, and breaks down the issue in as simple a way as I have seen, yet captures the fullness of the debate pretty well.
The First Amendment clearly states that the U.S. government cannot pass any laws establishing a national religion or giving preference to one faith over another. Preventing gay marriage would do exactly that. The founders of this nation were well aware of the private-public divide and the issues upon which the government rightfully could act. Same-sex marriage is not one of them.
The notion that same-sex marriage is morally wrong is essentially a religious conclusion. By proceeding from such a conclusion and enacting laws that, in essence, recognize the correctness of that opinion, the U.S. government would be integrating a theological argument into its constitutionally-mandated secular legal structure. Previously passed laws such as the Federal Defense of Marriage Act, or DOMA, of 1996 openly violate these provisions of the Constitution.
This notion,
The notion that same-sex marriage is morally wrong is essentially a religious conclusion.
is both right on the money, and yet also indicative of the larger problem. I participated in a lengthy dialogue on a Catholic blog, about the the Catholic teaching and homosexuality. From that perspective and following that enlightening experience, I completely agree with the statement. Yet, my experience also shows me how successful the anti-gay forces have been in their attempt to define being gay as a choice and a lifestyle. In my opinion, it doesn’t matter what laws we pass- as long as enough people buy into that falsehood, the future for LGBT equality will be a rocky one.