Break up with her if her sexual history isn't jiving with your sensibilities. There is nothing wrong with not wanting a relationship with someone who isn't compatible with you in morals (of your particular making) or who you feel hasn't similar personal or relationship boundaries.
It's only been a month so back out now. I suggest in future that you get the sexual history question out in the open quickly (which you seem to have done here) and if that history is a deal breaker for you then you just break the deal.
I've found it's a waste of typing to try and tell someone that they are insecure just because they don't like that someone is too free sexually then they themselves are. I don't think it has much to do with being secure or confident but rather it's just a personal boundary that he/she is intitled to. Find someone with the same sexual sensibilities as yourself and you'll be partnering up with someone who is more compatible to yourself.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion