0721, 07/21 or 21st of July is a pun mainly seen in online japanese circles. When read outloud, 0721 can be pronounced as "Onanii" - which means matsurbation.
Coomer 1: Yo! Did you see Asanagi's illustration for 0721 today?
Coomer 2: Hell yeah it was hot as fuck.
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi