The problem is that 100+ years ago when state constitutions were drafted it never occurred to ANYBODY that it would EVER be necessary to define marriage as being between a man and a woman. Some states lucked out because when they dealt with bigamy they worded the law as being one man and one woman, but NOBODY envisioned a day when homosexuals would even be public about their perversions, much less asking to be married.
In Iowa, the state Supreme Court imposed homosexual marriage through court order. It was not done through the legislative process.