In the nationally representative General Social Survey, U.S. Adults (N = 33,380) in 2000–2012 (vs. the 1970s and 1980s) had more sexual partners, were more likely to have had sex with a casual date or pickup or an acquaintance, and were more accepting of most non-marital sex (premarital sex, teen sex, and same-sex sexual activity, but not extramarital sex). The percentage who believed premarital sex among adults was “not wrong at all” was 29 % in the early 1970s, 42 % in the 1980s and 1990s, 49 % in the 2000s, and 58 % between 2010 and 2012. Mixed effects (hierarchical linear modeling) analyses separating time period, generation/birth cohort, and age showed that the trend toward greater sexual permissiveness was primarily due to generation. Acceptance of non-marital sex rose steadily between the G.I. generation (born 1901–1924) and Boomers (born 1946–1964), dipped slightly among early Generation X’ers (born 1965–1981), and then rose so that Millennials (also known as Gen Y or Generation Me, born 1982–1999) were the most accepting of non-marital sex. Number of sexual partners increased steadily between the G.I.s and 1960s-born GenX’ers and then dipped among Millennials to return to Boomer levels. The largest changes appeared among White men, with few changes among Black Americans. The results were discussed in the context of growing cultural individualism and rejection of traditional social rules in the U.S.