You act as if it is a huge coincidence when a man and a woman have sex and the result is a baby is born. Marriage is not only about procreation. Marriage is about the stability of the families into which children are born.
When once in a while a gay couple adopts a child, that is what it is, a special case. It does not render invalid the way the vast majority of people reason about marriage and children, namely in terms of a mother and a father. When a man and woman marry without any intention of having children, it does not change the fact that the reason marriage exists has to do with children.
None of which will change one iota if same-sex marriages come into being.