The sexual act has been 'decoupled' from the procreative act since forever. People have always had ways to avoid pregnancy (some more successful than others) and sexual acts have always entailed much more than procreation. Not just within our species; look at how bonobos use sex for purposes other than procreation.
And falling birth rates in developed countries are not because of this mythical 'decoupling' you write about. There are many reasons for people not having as many children today and those reasons have been trending for a long time before the invention of the modern birth-control pill.