Because YoonYul does.
“We are YoonYul”
Let’s make this a math theorem shall we?
The YoonYul theorem is a statement proving that YoonHyun, YulSic, and YulTi do not exist. That is, if Yuri is with Yoona, Yuri can not be with Jessica or Tiffany and Yoona can not be with Seohyeon. The YoonYul theorem also runs on correlation to the JeTi theorem (not to be confused with Jedi) used in upper level math.
Proof: UFO from 2009/1/19
Some Dude: What is YoonYul???
Yoona: Real… haha
Hahah, I would have owned in Calculus.
YoSooRi!!!! ❤ Best threesome since YoCharMi (Yossi, Charmi(Rika), Miki).