Almost agreed.
I agree the Constitution doesn't cover gay marriage, and at first glance it would seem to be a matter for each State.
However, if the Constitution or Supreme Court can rule against discrimination it would seem to be able to rule on various forms of discrimination. Preventing marriage is a form of discrimination.
It would seem that the Constitution, or the Supreme Court, would trump the States attempt to legislate one form of discrimination. It wouldn't matter if it were schools, buses, employment, or marriage.