[0xcafefood]

I think what you're proposing will happen, but over time. Birth control has separated sex from reproduction, for anyone with access to it and the desire to use it. So nature's former trick of making sex enjoyable in order to incentivize reproduction doesn't work as well any longer. The next trick will have to be an actual desire to have children, instead of a desire to have sex with the results also including children. That desire may be genetic or memetic (e.g. in the form of a religion), or most likely a synthesis of the two.