It is not judicial activism when the judges are following the letter of the law.
Iowa Constitution
Article XII, Section 1:
"This constitution shall be the supreme law of the state, and any law inconsistent therewith, shall be void."
Article I, Section 1:
"All men and women are, by nature, free and equal, and have certain inalienable rights--among which are those of enjoying and defending life and liberty, acquiring, possessing and protecting property, and pursuing and obtaining safety and happiness.
Article I, Section 6:
"All laws of a general nature shall have a uniform operation; the general assembly shall not grant to any citizen, or class of citizens, privileges or immunities, which, upon the same terms shall not equally belong to all citizens."
Being a legal institution in Iowa, marriage must be applied equally to all of its citizens.
Iowa General Assembly - Iowa Constitution