To legalize marriage between two people of the same sex would enshrine in the law the principle that mothers and fathers are interchangeable or irrelevant, and that marriage is essentially an institution about adults, not children; marriage would mean nothing more than giving adults recognition and benefits in their most significant relationship.Read the whole thing.
But marriage transitioned from being child oriented to adult oriented 50 years ago when the Pill was released. When the biological connection between hetero sexual relations and child bearing and rearing was technologically severed to an extremely high level of confidence, society as a whole ceased having any interest in who was having sex with whom. Society shouldn't have stopped caring, but we did.
More on that topic here.