Interesting post by Will Baude responding to an article in The American Conservative that Congress has the power nationally to ban gay marriage even without an FMA.
Austin Bramwell, the author of the article, argues:
Congress derives the power to pass such a statute from the Fourteenth Amendment . . . . Section 5 of the Fourteenth Amendment gives Congress "the power to enforce by appropriate legislation, the provisions of the article." It is well-settled that the Fourteenth Amendment protects the fundamental right to marry. States may not violate this right by redefining marriage as something other than it really is. Therefore, Congress can pass a statute underscoring the correct definition of marriage.
Baude notes a number of problems with this analysis, namely that Supreme Court precedent holds that it's the Court, not Congress that gets to define the substantive rights (and what is constitutionally necessary to secure their enforcement) under the 14th Amendment. Congress may then, once those rights are manifest, pass legislation to better protect them. In other words, if the Court disagrees that defining marriage to include same-sex couples violates the fundamental right to marry, then Congress may not legislatively ban gay marriage, pursuant to its 14th Amendment powers, on the grounds that such is necessary to protect this fundamental right.
Likewise in the Boerne case, Congress passed a law requiring that generally applicable laws that have the incidental effect of burdening religious practice be justified by a "compelling government interest" (they essentially tried to legislatively mandate the "strict scrutiny" std. of review for such laws) on the grounds that this was necessary to protect the "free exercise of religion," a right that the the 14th Amendment protects against state infringements. The problem with that law was a few years before, in the Smith case, the Court held that the strict-scrutiny/compelling government interest test was not necessary to protect the free exercise of religion from generally neutral laws that incidentally burden religious practice. So the Court struck down this law in Boerne.
I think the larger problem for such federalist-conservatives is the notion that "[i]t is well-settled that the Fourteenth Amendment protects the fundamental right to marry." Okay, sure there are a few cases that have stated this, notably Loving v. VA. A 14th Amendment originalist would probably rather argue that this Amendment properly has nothing to do with the right to marry, which is wholly a matter of states' rights.
If the right to marry is indeed a fundamental right, then we have to ask whether forbidding gay couples from marrying violates this fundamental right, just as it did for interracial couples in Loving. It could be argued that Loving dealt not only with a right to marry, but also with race discrimination, something clearly covered under the 14th Amendment. Moreover, if the right to marry includes same-sex couples, then why not, polygamy, etc.? The answer is just as Loving dealt with hybrid constitutional rights -- fundamental right to marry, plus racial discrimination -- so does gay marriage: fundamental right to marry, plus gender discrimination. It is also well settled constitutional law that gender distinctions trigger heightened scrutiny (the intermediate test, not strict scrutiny). And that would give the Court grounds to argue why the fundamental right to marry should be applied to interracial and same sex couplings, but not extended to polygamous and incestuous ones, which involve no plus rights (we don't have to worry about bestiality because animals have no right to contract in the first place).