Dale Carpenter makes it in a paper published at Cato's site.

Here's the executive summary:

Members of Congress have proposed a constitutional amendment preventing states from recognizing same-sex marriages. Proponents of the Federal Marriage Amendment claim that an amendment is needed immediately to prevent same-sex marriages from being forced on the nation. That fear is even more unfounded today than it was in 2004, when Congress last considered the FMA. The better view is that the policy debate on same-sex marriage should proceed in the 50 states . . . .

A person who opposes same-sex marriage on policy grounds can and should also oppose a constitutional amendment foreclosing it, on grounds of federalism, confidence that opponents will prevail without an amendment, or a belief that public policy issues should only rarely be determined at the constitutional level.

There are four main arguments against the FMA. First, a constitutional amendment is unnecessary because federal and state laws, combined with the present state of the relevant constitutional doctrines, already make court-ordered nationwide same-sex marriage unlikely for the foreseeable future. An amendment banning same-sex marriage is a solution in search of a problem.

Second, a constitutional amendment defining marriage would be a radical intrusion on the nation's founding commitment to federalism in an area traditionally reserved for state regulation, family law. There has been no showing that federalism has been unworkable in the area of family law.

Third, a constitutional amendment banning same-sex marriage would be an unprecedented form of amendment, cutting short an ongoing national debate over what privileges and benefits, if any, ought to be conferred on same-sex couples and preventing democratic processes from recognizing more individual rights.

Fourth, the amendment as proposed is constitutional overkill that reaches well beyond the stated concerns of its proponents, foreclosing not just courts but also state legislatures from recognizing same-sex marriages and perhaps other forms of legal support for same-sex relationships. Whatever one thinks of same-sex marriage as a matter of policy, no person who cares about our Constitution and public policy should support this unnecessary, radical, unprecedented, and overly broad departure from the nation's traditions and history.  

Andrew Sullivan chimes in:

You have in this a classic example of the distinction between conservatism and fundamentalism. Conservatism seeks to govern society as it is and as it evolves. It allows for diversity and federalism and local rule. Instead of demonizing minorities, conservatives seek ways to integrate and include them and foster responsibility among them. Fundamentalists, in contrast, begin with an a priori religious deduction - homosexuality is Biblically or "naturally" wrong and homosexuals as such do not exist - and proceed to enforce that view on everyone. If constitutional procedures or principles of federalism get in the way of such doctrinal truth, then those procedures and principles must be abandoned. There is no extreme they will not seek, because God demands it. You saw that in the Terri Schiavo case, where the Christianist right abandoned even a pretense of believing in federalism or the rule of law. And you see it in the case of the FMA. This battle is not just about gays. It's about the survival of limited government conservatism and inclusion within the Republican party.