General Social Survey, which is a pretty good source as these things go, Americans are having less sex. This is partly because the population is aging, and 80-year-olds are a generally chaste bunch.
The underlying cause is of course delayed marriage; fewer young people are having sex because fewer of them are married or otherwise partnered.
Does this maybe explain the country's sour mood? Are the forces that drive young people to delay marriage -- careerism, graduate school, extended adolescence -- making us cranky?