She's either faithful to you or not... there is no middle ground. She cannot be 'conditionally' faithful to you. She cannot be allowed to cheat when you're gone for 5 minutes... or allowed free access to other guys on Mondays, Wednesdays, and Saturdays... or else she's a 'friend with benefits' and not a girlfriend.
There is nothing wrong with expecting fidelity from your girlfriend, and it should be expected that infidelity is reason enough to end the relationship. What if she's sexually involved with someone who has an STD? What if she gets pregnant? Suddenly her problems from being a whore becomes your problems... and that isn't fair at all.
Cut her loose... she doesn't respect you or herself. If she continues on this path she will end up contracting some disease or enduring some other consequence. If you remain with her... you will end up paying for HER mistakes as well.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen