The 14th Amendment, § 4, first sentence, actually reads,
The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned.Note those pesky words "authorized by law," which keep getting left out. If I didn't know any better, I'd almost think that people avoid quoting those words because those words get in the way of the desired result. Who makes laws again? The next section of the amendment gives a clue:
The Congress shall have power to enforce, by appropriate legislation, the provisions of this article.