The unanimous Iowa marriage decision is based on the state's equal protection clause. Applying those principles, the court held that:
(1) As a threshold matter, gay couples are "similarly situated" to heterosexual couples with respect to their relationship commitment and capacity to benefit from state recognition.
(2) The exclusion of same-sex couples from marriage is a form of sexual-orientation discrimination, because it "closely correlates" with being homosexual.
(3) Sexual-orientation discrimination merits heightened judicial scrutiny.
(4) The heightened scrutiny applied here is "intermediate," requiring that the statute at issue be substantially related to an important state objective. (The court reserved the possibility that it might apply strict scrutiny in a future case.)
(5) There were five objectives advanced by the government: (a) tradition, (b) promoting the optimal environment for children, (c) promoting procreation, (d) promoting stability in opposite-sex relationships, and (e) preservation of state resources.
(6) The state's five objectives were either not sufficiently important (e.g., maintaining a "tradition" for the sake of tradition alone is a circular objective) or were not substantially advanced by excluding same-sex couples from marriage (e.g., excluding gay couples from marriage does not encourage heterosexual procreation or stabilize heterosexual relationships).
(7) Religious objections to same-sex marriage cannot control the definition of marriage, which is a "civil contract" under Iowa statute.
This is the third pro-SSM state supreme court decision in the past year. In addition to the important marriage result, the decision is notable because it continues a growing trend among state courts to treat sexual-orientation classifications as suspect. If it continues, that trend will have consequences on gay-rights questions well beyond the marriage context. State judiciaries are beginning to follow a familiar pattern of hastening civil-rights progress for a group once that group's cause has achieved a measure of legislative success and cultural acceptance.
No other state in the Midwest even recognizes same-sex domestic partnerships, much less civil unions, or marriages. Same-sex marriages will actually begin in Iowa in about three weeks. The state has no residency requirement for marriage, meaning that gay couples elsewhere in the Midwest can easily travel there and get married, although their relationships will not be recognized when they return to their home states. I can see two simultaneous effects from this: (1) rising expectations among gay couples in the Midwest combined with more political pressure to enact domestic partnerships and civil unions, especially in Illinois, and (2) rising alarm and political organizing among gay-marriage opponents in those same states.
The Des Moines Register has more on how the state is reacting. Among other things, the paper estimates that unless the legislature acts very quickly, the state's demanding constitutional amendment process means there would be no possibility of passing a state constitutional amendment to ban gay marriage until 2012.
Either way, there's a large political battle ahead.