I'm not surprised. It's an easy sale and I'm sure that the retailers didn't order nearly enough of them.
From Image Comics:
The hit book that has everyone talking, is causing headlines again. SEX #2, the second issue of the new Image Comics series by Joe Casey and Piotr Kowalski, has sold out of its initial printing at the distributor on the day of release. A second printing of SEX #2 is underway and will be available for sale on May 15, 2013.
SEX, the story of a former superhero who hangs up his cape to pursue the kind of life he had forbidden himself, has been positively received by fans and critics alike. SEX #1 sold out instantly and is already into a third printing. Seems like comic fans can't get enough of SEX!
SEX #2's second printing is available to order now (Diamond code FEB138644), and the third issue will be in stores on May 22.