"Without a rational relation to a legitimate governmental purpose, state-imposed inequality can find no refuge in our United States Constitution,"
It is a legitimate government purpose to promote gender inclusion in a foundational institution. Marriage is our earliest social institution that promotes and regulates the mixing of the sexes. SSM does the opposite it excludes and separates the sexes.
Additionally it is in the best interest of the child to be raised in a loving home by a mother and father who are happily married to each other. The government has an interest in promoting best practices when it comes to children.