You are on your way to become a Creep. You know, those guys who place value on women depending on how many sexual partners they've had and whatnot. You should stop thinking that way right now. It's ok to be sad (if you wanted to be her "first" as much as you wanted her to be your "first"), but it's NOT ok to be "disgusted" and "disappointed" at her just because she did a perfectly normal, perfectly natural thing. Most people are going to have sex with different persons in their lives, it's just the way it is. And there's nothing wrong with it, nothing disgusting or disappointing about it. You talk of her having sex as if she had killed a puppy or something. She just had sex, it's no big deal - if anything, you should be glad in case you ever do get together, because she won't be afraid of feeling pain and all that. It will be less awkward than it would have been if you two had both been virgins. It's a good thing.
Just ask her out already.