The inflection point of global population happened somewhere around 1962-1965 when it went from concave up (growth phase) to concave down (slowing phase). Population still continued to increase in many parts of the world, but it's the rate of change that slowed. In many ways, overall population is best modeled by a different equation, but the carrying capacity is variable including place-to-place, time, technology, food, economic and political concerns.
Even though population slowed, it still kept increasing. With a larger population, resource scarcity plus labor over-supply increased in first-world countries and so the ability to start families decreased. People who are relatively rich have an easier time having children than most people when the average purchasing power and average income is reduced. More inequality + more access to reproductive services -> fewer families -> fewer children.
The sexual revolution and The Pill happened around this time, increasing the number of divorces and reducing the number of formed family units. (Birth control isn't anything new, as Silphium was a cash-crop in the ancient world that was harvested to its extinction.)
Delaying marriage also reduces the number of births. It was the case in the 19th century that girls were having children at 15-16 through their late 20's, so family sizes were on the order of 6-10 children per couple.
One point that goes against the trend is modern divorce (with a prenup and in a non-community property state) allows a high-income earning man to start multiple families serially.
Women gained more access to the means of income earning, removing dependency on men. As such, many women's time and priorities were taken-up by work, and so starting a family was no longer a primary motivation.
Alternative lifestyle became more acceptable as population increased. The previous modality forced some people who were uncomfortable with heterosexuality to have families and children, but who wouldn't have passed on their genes to a new generation in the new modality. Furthermore, individuals in the modern era, not necessarily for sexual or gender reasons, adopted new and various subsets of lifestyles that also reduced and limited their reproductive success.
In conclusion, I think it's important for any wise person to not capriciously lash their identity to a random cult, external attribute or way that contains a lot of negative, judgmental/stereotypes or arbitrary "rules," and to use some common-sense, nuance and love rather that bitterness, scorn or hate.