Update 2: If you still can't access the Supreme Court's site, you can read the summary and opinion on the Des Moines Register's site.

The ruling came down to equal protection.

Under Iowa's tripartite system of government, courts give respect to the legislative process and presume its enactments are constitutional. The deference afforded to legislative policy-making is manifested in the level of scrutiny applied to review legislative action. In most equal protection cases, the court applies a very deferential standard known as the "rational basis test." Under this test, "[t]he plaintiff has the heavy burden of
showing the statute unconstitutional and must negate every reasonable basis upon which the classification may be sustained." Classifications based on race, alienage, or national origin and those affecting fundamental rights are, however, evaluated under a "strict scrutiny" standard. Classifications subject to strict scrutiny are presumptively invalid and must be narrowly tailored to serve a compelling governmental interest.

The court also recognized that an intermediate tier has been applied to statutes classifying persons on the basis of gender or illegitimacy. Under this level of scrutiny, a party seeking to uphold the statute must demonstrate the challenged classification is substantially related to the achievement of an important governmental objective.

Also, sexual orientation is now a suspect class now in Iowa:

In its analysis, the court found each factor supported a finding that classification by sexual orientation warranted a heightened scrutiny. The court, citing historical as well as present-day examples, concluded that gay and lesbian people as a group have long been the victim of purposeful and invidious discrimination because of their sexual orientation. There was no evidence that the characteristic that defines the members of this group--sexual orientation--bears any logical relationship to their ability to perform productively in society, either in familial relations or otherwise. Addressing the issue of immutability, the court found sexual orientation to be central to personal identity and that its alteration, if at all, could only be accomplished at the expense of significant damage to the individual's sense of self. This, the court concluded, would be wholly unacceptable for the government to require anyone to do. Finally, the court found that, despite their securing of significant legal protections against discrimination in recent years, gay and lesbian people have not become so politically powerful as to overcome the unfair and severe prejudice that produces discrimination based on sexual orientation.

Update 3:

Time for some actual Freeper commentary, presented for your amusement (they haven't had much time to explode, but it's starting):

Iowa

Iowa?

IOWA?

This is fully expected in the nutbar places like New England and California. But Iowa? This is about as disturbing a news article as I could have ever imagined.

Prop 8...make it a constitutional change and the black robes can't be run by the lavender mafia.

Ergo polygamy and everything else must now be allowed, too.

Hey wasn't Iowa ....BO's claim to fame !!
Can we scratch Iowa off the map ??

And OneNewsNow has a story up, with the quote marks around marriage, calling Lambda Legal a "New York-based homosexula rights organization."