Lyman Stone:

The decline in fertility is not due to women wanting fewer kids. It also isn’t due to men wanting fewer kids, or to population aging. Nor is it due to abortion: abortion rates are actually declining. It may be partly due to the rising usage of longer-acting contraception, or diminished sexual frequency, or any number of social factors. It might also be due to economic pinches on household budgets. But the truth is, none of those are probably the biggest driver of declining fertility. The decline in fertility is mostly due to declining marriage.