Over the years, Yuuri has gotten around. In a lot of skaters' beds. It was one of those things everyone knew about but never mentioned, that Katsuki Yuuri was an absolute fiend in the sack, and would not turn down anyone.
Well, everyone knew, except for Victor.
TD;DR Everyone knows Yuuri's slept with everyone except for Victor, who thinks Yuuri is a cute stuttering virgin. Cue shock and hilarity once Victor finds out from everyone but his fiance.