It's NORMAL to have a crush while u are in a
relationship...
Since a CRUSH is usually a temporary infatuation...
And an INFATUATION is a foolish, unreasoning, or extravagant
passion or attraction..
Eg: U see hot babe, u THINK u r in love with her, but when U
realise tt she laughs like a hyena... U think she's a big turn
off...
THAT's INFATUATION! TOTALLY diff from LOVE!
So why must u break up with your GF/BF becuz of sometin STUPID like
that?? 
Unless ur definition of crush means something else.... 