Thats easy...no sex.
Oh, there's plenty of sex going on, but it has become sterile through contraception, homosexuality, and now even through "sex dolls" and "sex robots" that is in reality a form of masturbation. If any conceived children slip through the cracks, they are snuffed out in their mother's wombs before they can take a breath. We live in a selfish and wicked society.
Studies say millennials are having less sex than previous generations, both in and out of marriage.