It doesn't answer the question you posed, but I read a study[1] just yesterday that estimated that the largest factor of young people having less casual sex now

It doesn't answer the question you posed, but I read a study[1] just yesterday that estimated that the largest factor of young people having less casual sex now than they did ten years ago is a reduction in drinking rates. It was still a minor factor (explains about 25% of the drop, by their estimation), but it seems less alcohol use does correlate with less casual sex. [1] https://journals.sagepub.com/doi/10.1177/2378023121996854 https://journals.sagepub.com/doi/10.1177/2378023121996854