Rather, when a family has one more child, each child in that family gets a lot less, with little effect on anyone else. If parents care about their kids, there is no externality. This is especially clear from bequests. I. Landsburg goes on to argue that there are positive externalities of child-bearing. Most people like being alive, but parents probably under-weight that when they weigh having another child.
Individuals try to get divorces when they decide they are better off without their spouse.
C. Complication - women's mate value generally falls more rapidly than men's. Alimony is one way to try to keep incentives well-aligned. In the pre-modern period, a husband's support was often crucial just to keep a child alive. When children get cheaper, women have more out of wedlock. One simple way to make them cheaper is to pay benefits proportional to the number of children a mother has - a frequent criticism of the welfare system.