I agree, but having a child is something that has an inherent, irrevocable power dynamic in it. The child cannot consent to being born. The parent's role is bound up by duty to the child.

Whereas in marriage, it's two adults, making a voluntary commitment to each other, each (hopefully) having thought through the potential consequences of their actions. Committing to another adult for the rest of your life is a different kind of love than being a parent.