perhaps religious and moral pressure not to excersize the choice available has decreased between 50s and 2000s

Cell-phones. Noone’s having sex because they’d rather watch Tik-Tok; it’s got lower friction than going outside.

Most of the drop occurred in the late 50s to mid 60s. Fertility in the US peaked at over 3.5 births per woman in the 50s before falling below 2 by the late 60s. It then stabilized for 40 years and even recovered to the low to mid 2s in the 90s and 2000s before falling post-2008 from 2.2 to 1.7.