The sex recession among young adults today relative to the young adults of the past is a consequence of the decline in the institution of marriage. The following table shows, by the decade the question was asked, the percentages of married and unmarried people between the ages of 18-29 who reported not having had sex at all over the past year (N = 5,852):
Unmarried young adults haven’t become more prudish over the last few decades, there are just more of them now than there used to be. Why has sexual activity decreased in total among the young if the rates of sexual activity among married and unmarried young adults has remained static? Because marriage has declined.
From the same data, the percentages of these young adult respondents who were married at the time of their survey participation:
1990s — 31.6%
2000s — 24.7%
2010s — 18.8%
GSS variables used: SEXFREQ, MARITAL(1)(2-5), AGE(18-29), YEAR(1990-1999)(2000-2009)(2010-2018)