I won't judge you for this view, as I personally can relate to what you're saying. My first wife had "experiences" that bothered me very much and to tell you the truth it affected my view of her.

The thing is, I don't know how you get over such things, but what I do see is not you being judgemental but a mismatch in values when it comes to sexuality. it's not that you're necessarily saying she's not as good as you are when it comes to how you view sex but that it's different and to be honest, it may be a source of problems now and in the future. Because her past will never change, and likely her casual approach to the sexual situations may never change either. It is not our right or our place to change them so it's up to us to accept or not accept the way they are.

So the ball is in your court. The problem stems from your feeling that she is promiscuous and of course this can make you fear her being faithful to you and may even affect your view of how "pure" she is sexually. Also if she feels sex is a casual thing, does she even value the sex she has with you? soemthing that unconsciously may be going through your head.

Unless you find ways to communicate to her your concerns, you won't be able to get over these things. Find non-accusatory ways to bring it up with her and give her a chance to calm your fears/concerns about this area.

Hope this helps
~S4