I maintain that people hold this view to make themselves feel better about butchering babies. On its face, ensoulment lacks empirical provability, so we err on the side of our convenience, since obviously it's not possible that "billions and billions of sentient souls [are] swarming the sewers and latrines of the world." IOW "I can't fathom it, therefore it cannot be true."
Contraception is not "butchering babies" IMO.