
There are a number of sources showing a decline in sexual frequency among American adolescents and adults. Here are a few trends that I gathered as I write the update for the 4th edition of my textbook.
Since 1989, the General Social Survey as asked people, “About how often did you have sex during the last 12 months?” They offer a series of categories, which I converted to a straight count of times per year for convenience, like this: Not at all (0 times), once or twice (1.5 times), once a month (12 times), 2 or 3 times a month (30 times), about once a week (52 times), 2 or 3 times a week (2.5 * 52 = 130 times), more than 3 times a week (5.5 * 52 = 286 times)
This is the trend, at the mean of controls for age, sex, and marital status:

This shows a decline of about 10 sexes per year since 2002. Unfortunately, the GSS only asks the question of two-thirds of its sample these days (since 2016), and the survey isn’t that big to being with, so it’s hard to get reliable breakdowns. But if you break that trend down by marital status, you can see that the decline is more clear for married people. If the 2021 data are to be believed, there was a big drop for single people in the first pandemic year. We’ll see if that continues in future years. (A breakdown by age shows the decline is greater among younger people, but it’s worth nothing the GSS does not include people in college dorms.)

There is also a decline among US high school students who report they have ever had sexual intercourse, by 10-15% since 1991, according to the latest Youth Risk Behavior Survey, collected by the Centers for Disease Control:
In an analysis of Americans age 18-23 in the Panel Study of Income Dynamics, Lei Lei and Scott South reported a drop of 10 points from 2007 to 2017 in the percentage who had sexual intercourse in the previous four weeks, from 59% to 49%. When they decomposed the trend, it was mostly attributable to fewer people being in romantic relationships, and drinking less alcohol, as well as earning less and playing computer games more.
We’ll have to wait for more data to assess the impact of the pandemic.
Of course, Americans are also having fewer babies nowadays, and these things may be related. Like the fertility trend (one analysis I did is here), there is a structure-agency story in these things. Some of the decline in sex would be great if it meant more people are avoiding sex they don’t want (and there’s some evidence for that in research on feminist attitudes within couples), some of it would be sad if it were the result of mental health problems like anxiety and depression (or antidepressant medication). I have no baseline for what is the optimal amount of sex in society, so up and down aren’t necessarily about good and bad, but it’s probably worth more scholarly attention.