No this is not relevant. Physical resilience to birth control is incredibly unlikely because the vast vast majority of people are born due to incorrectly applie

No this is not relevant. Physical resilience to birth control is incredibly unlikely because the vast vast majority of people are born due to incorrectly applied birth control or with no birth control at all (including purposefully of course). Most forms of birth control are extremely effective and our bodies have very limited abilities to evolve past them (especially condoms). Fitness is resilience to birth control offers no practical advantage. The desire to have kids is a social / cultural trait. We have plenty of evidence that it is fluid over time and changes with economic opportunities and whatever. It is very hard to maintain cultural cohesion in groups as they get larger. Even the Mormons would break ranks past some critical mass. The drop in fertility rate is due to changing culture and economic incentives. We have tons of data to support this and more of the world is headed in this direction economically and culturally.