"Jobs for women are higher-paying and more satisfying than ever before, and that raises the opportunity cost of having large families"
This is not true. Depending on the wife for income prevents her from focusing on having a large family.
You claim to be disagreeing but I think you are agreeing with that comment. By referring to "opportunity cost", the author means precisely that when a woman works it is harder to have a large family. Not having a job is one opportunity cost of children.