The following persons are not authorized to marry, and a town clerk shall not knowingly issue a civil marriage license, when:
(1) either party is under 18 years of age;
(2) either party lacks capacity to understand the nature of the conduct at issue;
(3) either party is 18 years of age or older and under guardianship, without the written consent of the party’s guardian;
(4) the parties are prohibited from marrying under 15 V.S.A. § 1a on account of consanguinity or affinity; or
(5) either party has a living spouse, as prohibited under 13 V.S.A. § 206.