Most Helpful Girl
"soolutt"? (is that slut?)
Say something along the lines of, "I'm sorry, but your sexual history makes me uncomfortable. And, I think you should be with somebody who accepts all of you, your past, your flaws (that might be a strong word), your everything. And, I know I can't be that for you, so I hope you can be happy." (this isn't anything toward you, it's perfectly okay to be bothered by it, but it's all true, she shouldn't be with somebody who is made uncomfortable by that, she deserves more than that, and frankly, so do you. And, I think it's important to wish her well. Because, she might have done something you don't approve of, but she deserves to be happy all the same. And, I think she'll understand, also.
Though, out of curiosity, is that she might not be clean, the amount of partners, what she's done, what makes you uncomfortable about it (don't mean this out of offense, I'm just a nosy little bugger)?