The court’s decision can be defended, in theory, on various grounds.
(1) The state’s constitution actually required the court to rule as it did.
I’m not going to debate this claim any more than I’m going to debate a Scientologist about whether we’re all really Thetans.
(2) The ruling is a “plausible” interpretation of the state’s equal protection clause and associated precedent. This is true. To say it’s plausible is to say it’s the kind of argument you can make in court, and judges might buy it. Note the claim upheld by the court would have been utterly implausible just a few (10? 20?) years ago, even though in the interim there’s been no change in the relevant formal legal materials. So what has changed? Obviously, the beliefs of judges. Just as obviously, this change in judicial opinion is a product of the change in the political landscape in regard to gay marriage.
All of which is to say that judges are now more likely to find a constitutional right to gay marriage than they were a generation ago (when the odds of them “finding” such a right could be calculated as zero) for exactly the same reason that legislators are now more likely to vote for civil union and gay marriage laws than they were a generation ago — because an idea that was politically unpopular has become more popular.
(3) It hasn’t become popular enough in Iowa., however, to be enacted through legislation. The defense of judicial review of this sort by people who aren’t sufficiently clueless to believe that what’s going on is the deduction of formally entailed legal conclusions from authoritative texts comes down to some combination of “it’s not really that anti-democratic for courts to do stuff like this because they need support from the political branches,” and/or “this is too important to be left to the normal political process.” I just want to note that these arguments tend to become somewhere between deeply implausible and utterly outrageous to those who make them whenever courts employ this type of judicial review to invalidate laws they like.
Now it’s certainly possible to defend aggressive judicial review under the guise of interpreting very general constitutional language. But two arguments that seem quite wrong to me are that such a practice is “legal interpretation” in any useful sense of that phrase, and that such a practice isn’t significantly less democratic than the typical legislative process (which of course itself is only very imperfectly democratic).