>porn
>feminism
>contraceptives/abortion
>culture in general
All irrelevant. These things are downstream from the real cause which is ECONOMIC.
>undeveloped sustenance agriculture society
Children are an INVESTMENT. There is a mountain of domestic work to be done that they can start doing at a young age. They are way cheaper than employing somebody else to work on the family farm/trade and there isn't a mountain of regulations preventing you from doing this. When you get old, they take care of you.
>highly developed post-industrial society
Children are an EXPENSE. What little domestic work there is left is done by machines and kids aren't even allowed to work actual jobs even if they could effectively. When you get old, the welfare state (someone else's children paying taxes) will take care of you.

The type of policies you'd need to seriously reverse this are so extreme it's hard to imagine the political appetite for them ever developing in our lifetimes.