We know testosterone levels are dropping: "After controlling for confounders—including year of study, age, race, BMI, comorbidity status, alcohol and smoking use, and level of physical activity—total testosterone was lower among men in the later (2011-2016) versus earlier (1999-2000) cycles (P https://www.urologytimes.com/view/testosterone-levels-show-s...
Setting aside that semen quality is also affected... Maybe testosterone, or other hormones that might be affected by the same thing, also affect the drive to have children? It seems the more urban a civilization, the lower the birth rate.
When I just tried to look up if anyone has looked into that I couldn't find anything, but here's a nice overview of different theories on fertility rates dropping as a whole: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8977063/