What reality?
If religion defines marriage then there is no definition, because there is no defined religion.
God defines all non-empirical reality - apart from God there is no such thing. And God has defined marriage as the union between a man and a woman. That is an unalterable fact, regardless of what some "religion" may say.
True that, unless the religion (or religions) in question define marriage in a way that's compatible with Natural Law, which is at the root of our Constitution.
An aspect of Natural Law applicable here, would be the children's right to receive from their natural parents their identity, their place in a kinship system, and their claims to nurturance and support until they reach their majority.