No, the cause of low fertility is not “well known”.

Furthermore, brothels and prostitution were extremely prevalent and accepted in the US and Europe up until the progressive era i.e the 1920s.

Your conception of historical marital and sexual norms is simply wrong.

You can tell when someone is an NPC when they’re quick to call someone a bot when confronted with heterodox ideas. The issue is this is beyond your intellectual abilities to contend with, or really your humility.