You're ignoring the alternate method the Constitution provides for ratifying an amendment. Congress can specify that ratification is to be by state legislatures, as it usually does, but it can only specify that it is to be by state conventions. In the one case where the latter method was adopted, the 21st Amendment, the conventions were popularly elected, and ratification was accomplished within the short period of around nine months. I think it is most unlikely that the necessary 1/4 + 1 of state conventions would be elected that would be needed to block ratification of a federal marriage amendment.