According to a recent study published in the Archives of Sexual Behavior, a large general social survey found that American adults had sex about nine fewer times per year in the early 2010s than they did in the late 1990s, a decline that wasn't explained by longer work hours or increased use of pornography.From CNN.
The theories about why this is happening in the linked article are basically rubbish.
A more plausible theory is that (1) there is lots of empirical evidence to show that married people have sex much more frequently than people who are not married, and (2) marriage rates have been falling significantly in the relevant time period (divorce rates have fallen too in that time period, but less quickly). This hypothesis is supported by the fact that the magnitude of the reduction in the rate at which people are having sex is consistent with what one would expect from this hypothesis.
Why do married people have more sex?
Mostly, because a lot less effort goes into making it happen. You have a partner and a place to have sex lined up all the time when you are married. But, when you aren't, considerable effort has to be expended to do so. Cohabiting couples may be similarly situated, but cohabitation is less stable than marriage, on average. So, people who aren't married spend a smaller percentage of the time, on average, not cohabiting with a sexual partner than married people do.