This blog post on the Iowa gay marriage decision is useful because of the legal background in it. Here's the critical part:
"What sets the Iowa opinion apart is a passage near the end. After dealing with the arguments the state offered to justify the exclusion of same-sex couples, the Iowa Supreme Court addresses head-on what it says is the “unspoken” reason many support the exclusion: religious opposition.
The Court says that while many oppose marriage for religious reasons, religion cannot justify a law excluding gay people from marriage. “State government can have no religious views,” the opinion says, “either directly or indirectly expressed through its legislation.” “This proposition,” the Court goes on to say, “is the essence of the separation of church and state.”