Declining Birth Rates, Populations, Sex, & Marriages

Why is it that when a country becomes "developed" its population begins to decline? Why is it that in "liberated" societies where women's rights are strong, we see lower birth rates? Why is population decline seen as "progress"? And what is it about "developed" societies that is detrimental to the flourishing of the human species?   The theory Ben references about delayed gratification is called I–D Compensation Theory by Leonard L. Martin. You can read about it here: https://psyc.franklin.uga.edu/sites/default/files/CVs/I-D%20Compensation%20Theory.pdf   The article Ben references about sex can be found here: https://www.scientificamerican.com/article/people-have-been-having-less-sex-whether-theyre-teenagers-or-40-somethings/