Here’s a fun formula from XKCD:

This is for the following variables:

Pd = regional population density in square miles
Xf = average person’s frequency of sex per year
Xd = average duration of sex

Then you solve for r, the average distance away from you (in miles) that someone is having sex at any given time.

I solved this for Harrisonburg, Virginia. At this moment, someone 618 meters away from me is neglecting his homework.

Want to solve this for your town? Here’s some help to get you started – according to a 2001 paper by Grenier and Byers, the average duration of the act itself is 7.9 minutes. Additionally, an average 18-29 year old has sex 112 times per year according to the Kinsey Institute. You can also save yourself the trouble of the minutes-to-years conversion for Xd by changing the numerator from 2 to 1,051,896 (this is derived from 2 x 525,948 minutes in a year).

Have fun!