Cheating on your partner with a girl???
No doubt being with a girl for sex would be quite exciting and as far as that goes there isn't anything wrong but as far as the cheating bit goes, there could be a lot wrong with that...
How important is your relationship to you?
How would you feel if your partner was doing the same thing with another girl, or even another man?
If you want to risk it all then go ahead but I strongly suggest you stick to those fantasies and make them work for you...