No it is not... 98 U.S. 145, REYNOLDS v. UNITED STATES. October Term, 1878.
And just in case you want to erroneously invoke the Costitution...
Article V
The Congress, whenever two thirds of both houses shall deem it necessary, shall propose amendments to this Constitution, or, on the application of the legislatures of two thirds of the several states, shall call a convention for proposing amendments, which, in either case, shall be valid to all intents and purposes, as part of this Constitution, when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof, as the one or the other mode of ratification may be proposed by the Congress;...
The Correct Supreme Court precedent to cite in reference to "gay" marriage is one that the homo-activists have tried to suppress. It's a bit complicated, and because it is so, I'll let Justice Kennard of the California Supreme Court explain the detail to you, for me:
[i]Indeed, there is a decision of the United States Supreme Court, binding on all other courts and public officials, that a state law restricting marriage to opposite-sex couples does not violate the federal Constitution’s guarantees of equal protection and due process of law. After the Minnesota Supreme Court held that Minnesota laws preventing marriages between persons of the same sex did not violate the equal protection or due process clauses of the United States Constitution (Baker v. Nelson (Minn. 1971) 191 N.W.2d 185), the decision was appealed to the United States Supreme Court, as federal law then permitted (see 28 U.S.C. former § 1257(2), 62 Stat. 929 as amended by 84 Stat. 590). The high court later dismissed that appeal “for want of substantial federal question.” (Baker v. Nelson (1972) 409 U.S. 810.) As the United States Supreme Court has explained, a dismissal on the ground that an appeal presents no substantial federal question is a decision on the merits of the case, establishing that the lower court’s decision on the issues of federal law was correct. (Mandel v. Bradley (1977) 432 U.S. 173, 176; Hicks v. Miranda (1975) 422 U.S. 332, 344.) Summary decisions of this kind “prevent lower courts from coming to opposite conclusions on the precise issues presented and necessarily decided by those actions.” (Mandel v. Bradley, supra, at p. 176.) Thus, the high court’s summary decision in Baker v. Nelson, supra, 409 U.S. 810, prevents lower courts and public officials from coming to the conclusion that a state law barring marriage between persons of the same sex violates the equal protection or due process guarantees of the United States Constitution. The binding force of a summary decision on the merits continues until the high court instructs otherwise. (Hicks v. Miranda, supra, 422 U.S. at p. 344.) That court may release lower courts from the binding effect of one of its decisions on the merits either by expressly overruling that decision or through “ ‘doctrinal developments’ ” that are necessarily incompatible with that decision. (Id. at p. 344.) The United States Supreme Court has not expressly overruled Baker v. Nelson, supra, 409 U.S. 810, nor do any of its later decisions contain doctrinal developments that are necessarily incompatible with that decision... Until the United States Supreme Court says otherwise, which it has not yet done, Baker v. Nelson defines federal constitutional law on the question whether a state may deny same-sex couples the right to marry."[/i]
Baker v Nelson, although it's only a summary decision, is binding precedent to this day, and is the LAW OF THE LAND.
And yes, Marriage is a STATE issue. I could cite case after case that proves this, but I don't need to. It's settled law. What you are probably concerned about is the Full Faith and Credit Clause. However, this fear may be a bit misplaced. A VALID reading of the FF&C Clause would not mandate that other States accept the marriage definition of another.
There are three reasons for this. 1st, Marriage Licenses don't reach the level of a Judgment (while strangely a divorce does) under the clause. 2nd, there is a public policy exception to the clause, and 3rd, Congress has the express power to define how the clause works, and this is something that they did with the 1996 DOMA.
The only concern is that an activist and out of control court would rewrite the law and settled precedent from the bench. I believe that this is a valid concern, and thus is more than enough reason to push for the FMA.
Marriage is a State issue. And a valid interpretation of FF&C wouldn't put other states at risk of having to accept an out of state homo "marriage".
That doesn't mean that we need to support a homo-loving RINO for President though.