Natural selection will fix this in no time, as the genes and cultures that lead to people not making kids die off. Widespread availability of contraceptives and abortion is recent enough that we just haven't had the time to adapt yet. The desire to have sex has been enough to keep birth rate high for most of human history. Now evolution is strongly selecting for cultures and genes that lead to more kids even in the presence of birth control. In a few of generations we'll start seeing birth rates recover.