What I said above just applies to the definition of equal protection as it is stated, for example, in Black's Law Dictionary. So I assumed that, to the extent the Iowa Supreme Court differed from that, they were in error.
After your highly rational and logical response, I actually went and read the opinion (if you think it was a 30 second read, you obviously haven't even seen it) and sure enough the court writhed and contorted and came up with an equal protection issue to justify their decision. There was a lot rationalization in there as to how the public benefits from stabilizing gay relationships, which may be true but has nothing to do with equal protection. As I see it, they were wrong.
I still maintain what I said earlier: There is no equal protection issue as to gay marriage. Once again, I have no objection to gay marriage, but I don't think it's a constitutionally guaranteed right, even in Iowa. I still haven't read the Iowa Constitution, but the court referenced the pertinent sections in the opinion and I didn't see anything it that would differ from the U.S. Constitution as to equal protection.