Marriage between one man and one woman is based on the "reproductive model". That doesn't mean a married couple must have children together. It means that only a man and a woman CAN have children together. Only a man and a woman can procreate.
State marriage laws always mention children. Yes, some couples are deemed unlikely to bear children, while others plan not to have children. But, sometimes those couples unexpectedly DO have children. And, in the event that they do, their biological family unit is protected by the legally recognized institution of marriage.
OTOH, two people of the same sex can never procreate.
I guess I am thinking of retired folks, widows and widowers and such, marrying. We all know there is no way they are going to have kids. I just feel like the “procreation” argument sort of shuts them out.