Responses
While I do think the explanation is rushed and a bit lacking, they did allude to it by mentioning how a three person household has a higher income and child rearing is easier with 3 parents... Which are both important factors to build a family. I think the idea is not "more partners = more babys" but that just having children is made more attractive again.

Love how the reasoning is the birth rate but you only get references to two husbands instead of two wives. Wouldn’t two wives make more sense lmao