Sadly, most of this decline is because of an upswing in the use of chemical abortion known as the “morning after” pill. Over 20% of all women had used it as of 2015. God only knows what the statistic is now.
Perhaps also,
Sperm Counts Continue to Fall
https://www.theatlantic.com/family/archive/2018/10/sperm-counts-continue-to-fall/572794/