The conservatives who oppose this keep changing their argument. It's confusing.
First, it was about "think of the children", when they were spreading fear that kids would be "indoctrinated at school by gay teachers". Now, it's about defining marriage being between one man and one woman. Then they rally around the notion (again) that man + woman = children. So it's back to "think of the children". I've even heard some talk that traditional marriage people fear being persecuted if gay marriage is legal.