Depends on your own personal ethics and current situation. You've got a number of options.

1) Don't cheat on her.
2) If you're already cheating on her, park your car in an out of the way location.
If you're not cheating on her, and you're not the type to date people who can't take betrayal without physical violence to you and your stuff then dump her...and refer back to #2 in regards to your car.