“You do not need to recognize marriage as a God-ordained institution to recognize that 100% of children come from heterosexual unions. Without that fact the government would have no interest in any kind of marriage”
Untrue.
Every society has rules to channel and regulate sexual expression, for reasons that should be obvious.
Indissoluble man-woman pair bonding, which all human societies have I common, is an important component of this universal phenomenon.
I recognize it as God-orgained. But it is not required to recognize that in order to recognize that man-woman unions are special.
In the same way the heavens declare the glory of God, nature declares the uniqueness and specialness of heterosexuality through the children it produces. God wrote it into our genetic code and it can be seen whether we acknowledge Him or not.