Most Helpful Girl
Only if there was a specific mutual agreement that it's okay to do so- in which case, it wouldn't really be cheating.
I'm guessing you're not being serious. But on the off chance that you are: Break up with your girlfriend and find someone else who is more sexually compatible with you. It's not okay to go behind someone's back that way. Cheating is extremely disrespectful and low, and is a huge betrayal of trust. It's always better to end a relationship that isn't making you happy than to go down that road.