The Supreme Court only needs to rule that the definition of marriage is a state issue and that the federal courts have no authority to overrule state legislatures and constitutions.
In theory you are correct.
But given the membership of this Supreme Court, I doubt they will rule that marriage is a state issue to be dealt with only by state courts or state legislatures. Not when federal courts have imposed homosexual marriage on so many states, and have declined to intervene, to prevent homosexual marriages from occuring.