*In the west.

I think gender equality may be a big culprit. Take the US for example: women will dominate the college educated class soon if they haven't already, and with that economic freedom comes more optionality in choosing a partner, there are more choices to be had and there will be a self selection of the most desirable men (typically college educated as it correlates to income).

This is not to blame women in any sense, it's a great thing that we're achieving a more equal society, but with a industrial well educated society more optionality with partners will lead to less sex imo.