Art. III, Section 2, in part:
In all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party, the supreme Court shall have original Jurisdiction. In all the other Cases before mentioned, the supreme Court shall have appellate Jurisdiction, both as to Law and Fact, with such Exceptions, and under such Regulations as the Congress shall make.
Congress has the power to remove issues from the Supreme Court. In other words they can say that the Court may not make decisions on a law they pass regarding marriage.
Is that where that Congressional power comes from? I have never looked it up but have heard both liberal and conservatives admit it is so.