Any adult couple could enter into a civil union

Vermont has done this and it has been interpreted as marriage by all the knee-jerks. And raised questions about Article IV Sect. 1 of the US Constitution:

Article IV

Section 1. Full faith and credit shall be given in each state to the public acts, records, and judicial proceedings of every other state. And the Congress may by general laws prescribe the manner in which such acts, records, and proceedings shall be proved, and the effect thereof.