No time for a an episode review, but some capsule material anyway:
Utah's marriage laws pertaining to references made in this episode:
30-1-2
The following marriages are prohibited and declared void:
(1) when there is a husband or wife living, from whom the
person marrying has not been divorced;
(2) when the male or female is under 18 years of age unless
consent is obtained as provided in Section 30-1-9;
(3) when the male or female is under 14 years of age or,
beginning May 3, 1999, when the male or female is under 16 years of age
at the time the parties attempt to enter into the marriage; however,
exceptions may be made for a person 15 years of age, under conditions
set in accordance with Section 30-1-9;
[...]So, Bart would not have been able to get married as the minimum age (with restrictions) is 15. And polygamous marriages are also not allowed by subsection (1).
Where is Springfield? --Springfield is not in Utah, as that's where Bart and Darcy have to travel --Springfield is not in Colorado, as Homer orders "Rocky Mountain Chew" from Colorado --Bart's imagination seems to know a lot about Utah's geographic features for a ten-year-old "underachiever". Perhaps this, along with the apparent relatively short drive places Springfield near Utah or at least somewhere in the Western US.