from SF Gate:
...Frequently citing the California ruling, the Iowa justices said any law that denies equal treatment to a historically persecuted group, like homosexuals, is valid under the state Constitution only if it promotes an important, legitimate government goal. They said none of the goals cited in defense of the marriage law - tradition, protecting children, encouraging procreation - met that test.
Excluding a group from marriage merely because of long-standing custom "can allow discrimination to become acceptable as tradition," said Justice Mark Cady, one of two Republican appointees on the court. Claims that children are better off with opposite-sex parents are scientifically unproven, he said, and, even if true, would not be served by denying marriage to same-sex couples who are already raising children.
Likewise, Cady said, neither evidence nor common sense supports the notion that limiting marriage to opposite-sex couples would encourage those couples to have more children or make their unions more stable.
Much like the California court, which invoked its landmark 1948 ruling legalizing interracial marriage, the Iowa court looked to history - its 1839 ruling against slavery, its rulings in 1868 and 1873 against racial segregation, and the state's pioneering 1869 decision to let women practice law - as precedent for maintaining a "path ... navigated with the compass of equality firmly in hand."
As for the option of allowing same-sex couples to form civil unions, with the same rights as spouses under state law, Cady - like the California court - said a separate status, created for gays and lesbians, is also inherently discriminatory.
The ruling illuminated a difference in the political structures of the two states: Iowans, unlike Californians, cannot amend the state Constitution by initiative... hm, perhaps I should find a writeup that isn't quite so Prop-8-centric.