I can think of, at least, another possible explanation. Before birth control was available, there is not need for evolutionary effects in the willingness to ha

I can think of, at least, another possible explanation. Before birth control was available, there is not need for evolutionary effects in the willingness to have children directly, only for the willingness to have sex. You probably agree with me that sexual desire is pretty common, obviously we have evolved for it. After birth control exist, sexual desire is not an appropriate proxy for evolutionary pressures, so, we could see an increase in the instinct to have children and, maybe, a decrease in the instinct of having a lot of sex.