That's possible but I'm not sure why someone who would prefer more descendants would genetically modify their progeny to not prefer more descendants. If this is a government imposed eugenics plan it could certainly work, though.
For most of human history a desire for sex was quite sufficient to ensure reproduction even without as much desire for children. With the advent of birth control the environment changed and there are new selective pressures. Also the modern world has a lot of new, competing, sources of joy.
R strategies certainly don't always beat K strategies! But humans are currently in a very rare situation where our population seems to be very far below K and most people in wealthy countries aren't living anywhere near subsistence level. So, until Malthus rears his head again, evolution is going to be paying a lot more attention to r.
For those following along at home, for N as the number of individuals:
dN/dt = rN(1 - N/K)
