This trend is probably the best example possible of "correlation does not imply causation".

If there is any clear reason at all, it's simply that sex is a choice. It has been completely unrelated to survival for a long while now. Yes, it is technically required at scale that someone has babies, but if no particular individual feels that pressure you should expect it to fluctuate like any other choice.

Sex would have to mean something to the individual for the numbers to go back up.