If parents are getting divorced because they are arguing about the upbringing of girls, why is this risk only materializing when the girl is the first child? Wh

If parents are getting divorced because they are arguing about the upbringing of girls, why is this risk only materializing when the girl is the first child? Why would the dynamics be different if the girl was not the first child? Ergo, the conclusion is not thought through and likely to be false.