I was recently reading Sexual Behavior in the United States: Results from a National Probability Sample of Men and Women Ages 14–94. At N ~ 6,000 it's a large sample of American sexual behavior around 2010. There was one descriptive result which I thought was interesting, though not surprising. Before the age of 25 it seems that women are more likely to have sex in a given year than an equivalent age man. After the age of 25 this starts to reverse, and men are more likely to be having sexual intercourse in a given year. The dynamics underlying this phenomenon seem to be easily subject to various speculations, so I'll leave that to readers. Rather, I offer the graph (data drawn from the paper linked above):
