The justices cited equal protection in their decision, and also noted that objections to same-sex marriage are purely religious in nature:
In one part of the decision that focuses on religious opposition to same-sex marriage, the justices seemed to anticipate negative reactions, saying they considered the unspoken reason for the ban on same-sex marriages to be religiously motivated. The justices said marriage was a “civil contract” and should not affect religious doctrine or views.
Thank you, Iowa Supreme Court, for saying exactly what I've said for years: marriage as regulated by the government is a secular institution. If two heterosexual atheists are allowed to get married, then the civil marriage contract cannot be a religious contract. Therefore, religious arguments against any type of marriage -- mixed-race, same-sex, ugly people, you name it -- are irrelevant to the practice of marriage contracts as regulated by the government.
If your church doesn't want to marry two people of the same sex, that's fine -- don't do it. Nobody can force you to, just like they can't force you to sanctify the marriage of atheists in the church, or people of two different races, or whatever other couples who break your religious tenets. But your religion does not get to inform the government of the fitness of couples to be joined in civil marriage. No way.
Thank you, Iowa Supreme Court, for getting it. You are made of awesome!