Despite the title this OVA has nothing to do with sex, just young people rebelling against the world. 
 
The promotional video is a home video exclusive titled SEX ~prologue~ directed by Sayo Yamamoto and animated by studio MAPPA. The anime depicts an original story before the manga where Kaho arrives, and when Yuki and Natsu have just met in Okinawa. 
 
To receive the PV, customers must first purchase all four volumes of the Sex 30th Anniversary Edition manga volumes, take the tickets included in volumes 1-3 and send them with the postcard included with volume 4. The Blu-ray Disc will be sent to buyers in March 2018 so long as the postcard is postmarked before December 31. 
 
(Source: Anime News Network, edited)
 
 Note: Had pre-screenings during March 8 to 14th, 2018 at the Liberty Gallery's Atsushi Kamijo solo exhibition.

SEX: prologue