A product of the time, similar to the prohibition on "spilling seed".
If you're a small tribe facing much more numerous foes, you naturally want to increase your numbers as quickly as possible. Anything which detracts from population growth is "wrong".
Whereas today, there are too many people in the world. Unless you want to massively increase the death rate, the birth rate HAS to come down.
And if we don't do it voluntarily, Nature will take care of it in the form of starvation, disease, and war.