I can certainly appreciate that you have a different view or opinion about casual sex than your current partner but acknowledge it for what it is. A difference. If you can't be with her because you don't share the same view, than cut it off.
Having similar opinions on certain things can be very important in a relationship. I couldn't be with someone who is racist. I just disagree with that too strongly.
Maybe she is having the same struggle knowing she is with someone who is a bit judgemental or prudish or whatever the opposite is...
Maybe for you, the fact that someone has had casual sex is a deal breaker. Nothing wrong with that. Maybe its just that important to you. You can only decide that for yourself. There's no right or wrong reason, its just how you feel. Don't try to justify it or be ok with it if you're not.
But I do think it worth it for you to know that it may hurt her feelings to think that you are feeling superior or that she is immoral when its simply a difference of opinion. I personally agree with danny's girl that you should not punish her for decisions she made pre-you.
I happen to have had promiscuous 20's. I disagree that it was damaging. I am happily married, my husband knows I had sex before him, its a complete non-issue. Maybe he's comfortable enough with himself and just likes me because I'm a nice person. Regardless of sex. Having had sex before I was married or just because I wanted to does not make me immoral, untrustworthy or less of anything. I'm a professional person raising a family in a middle class suburb. What does having sex or waiting or sexual preference have to do with anything?
And on a personal note, I am a firm believer in the difference between sex and love. Yes the two go well together, like broccoli and cheese. Together very good and contain things that are good for you. But both have good things separately too. And sometimes, even with my husband, I just wanna forget about the vegetable and go straight for the cheese.
Sex is part of our nature. We're sexual creatures. You can moralize it all you want....stifle it, call it wrong, put limits and laws on it, forbid it, ridicule those who have it, but you'll never get rid of it. None of us ('cept maybe the clones) would be here without it.
Just try to be ok with the fact that there are different people in the world. If you can't, than find someone who has views closer to your own. Or clone yourself....