@thefinn Serfs / peasants reproduced because there were no other options. If you knocked a girl up you married her - or else. With no birth control you were gonna knock her up eventually if you were in a 'relationship'. The church provided a single channel for lust - marriage. That's how kings and lords kept the labor supply up.
Now non-marital sex is celebrated, marriage does not exist thanks to no-fault divorce and birth control is available over the counter everywhere. For women who do want children the state is ready to foot the bill or force the abandoned father to do so.
It turns out the maternal instinct is very weak.